Image decoding method based on chroma quantization parameter data, and apparatus therefor

ABSTRACT

An image decoding method performed by a decoding apparatus according to the present document comprises the steps of: obtaining a flag indicating whether quantization parameter data for combined chroma coding is present on the basis of a type of chroma; obtaining the quantization parameter data for the combined chroma coding on the basis of the flag; deriving a chroma quantization parameter table on the basis of the quantization parameter data; deriving a quantization parameter for the combined chroma coding on the basis of the chroma quantization parameter table; deriving residual samples on the basis of the quantization parameter; and generating a reconstructed picture on the basis of the residual samples.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/546,507, filed on Dec. 9, 2021, which is a continuation pursuant to35 U.S.C. § 119(e) of International Application PCT/KR2020/007584, withan international filing date of Jun. 11, 2020, which claims the benefitof U.S. Provisional Patent Application No. 62/860,237, filed on Jun. 11,2019, the contents of which are hereby incorporated by reference hereinin their entirety.

BACKGROUND OF THE DISCLOSURE Field of the Disclosure

This document relates to an image coding technology and, moreparticularly, to an image decoding method of coding image informationbased on chroma quantization parameter data for a chroma component in animage coding system and an apparatus thereof.

Related Art

Recently, demand for high-resolution, high-quality images, such as HighDefinition (HD) images and Ultra High Definition (UHD) images, has beenincreasing in various fields. As the image data has high resolution andhigh quality, the amount of information or bits to be transmittedincreases relative to the legacy image data. Therefore, when image datais transmitted using a medium such as a conventional wired/wirelessbroadband line or image data is stored using an existing storage medium,the transmission cost and the storage cost thereof are increased.

Accordingly, there is a need for a highly efficient image compressiontechnique for effectively transmitting, storing, and reproducinginformation of high-resolution and high-quality images.

SUMMARY

A technical object of the present disclosure is to provide a method andapparatus for improving image coding efficiency.

Another object of this document is to provide a method and apparatus forimproving efficiency of data coding for quantization parameterderivation for a chroma component.

According to an embodiment of this document, there is provided an imagedecoding method performed by a decoding apparatus. The method includesobtaining a flag representing whether quantization parameter data forcombined chroma coding is present, based on a chroma type, obtaining thequantization parameter data for the combined chroma coding based on theflag, deriving a chroma quantization parameter table based on thequantization parameter data, deriving a quantization parameter for thecombined chroma coding based on the chroma quantization parameter table,deriving residual samples based on the quantization parameter, andgenerating a reconstructed picture based on the residual samples.

According to another embodiment of this document, there is provided adecoding apparatus performing image decoding. The decoding apparatusincludes an entropy decoder configured to obtain a flag representingwhether quantization parameter data for combined chroma coding ispresent, based on a chroma type and to obtain the quantization parameterdata for the combined chroma coding based on the flag, a residualprocessor configured to derive a chroma quantization parameter tablebased on the quantization parameter data, to derive a quantizationparameter for the combined chroma coding based on the chromaquantization parameter table, to derive residual samples based on thequantization parameter, and an adder configured to generate areconstructed picture based on the residual samples.

According to still another embodiment of this document, there isprovided a video encoding method performed by an encoding apparatus. Themethod includes deriving residual samples for a chroma component,generating quantization parameter data for combined chroma coding of theresidual samples based on a chroma type of the chroma component,generating a flag representing whether the quantization parameter datafor the combined chroma coding is present, and encoding the quantizationparameter data and the flag.

According to still another embodiment of this document, there isprovided a video encoding apparatus. The encoding apparatus includes aresidual processor configured to derive residual samples for a chromacomponent, to generate quantization parameter data for combined chromacoding of the residual samples based on a chroma type of the chromacomponent and to generate a flag representing whether the quantizationparameter data for the combined chroma coding is present, and an entropyencoder configured to encode the quantization parameter data and theflag.

Advantageous Effects

According to the present disclosure, a quantization parameter table forquantization parameter derivation can be determined based on a flagrepresenting whether quantization parameter data for deriving aquantization parameter for a chroma component has been transmitted, andcoding efficiency can be improved by performing coding based on aquantization parameter according to characteristics of an image.

According to the present disclosure, a quantization parameter table fora chroma component can be determined based on signaled chromaquantization data, and coding efficiency can be improved by performingcoding based on a quantization parameter according to characteristics ofan image.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 briefly illustrates an example of a video/image coding device towhich embodiments of the present disclosure are applicable.

FIG. 2 is a schematic diagram illustrating a configuration of avideo/image encoding apparatus to which the embodiment(s) of the presentdisclosure may be applied.

FIG. 3 is a schematic diagram illustrating a configuration of avideo/image decoding apparatus to which the embodiment(s) of the presentdisclosure may be applied.

FIG. 4 schematically shows an image encoding method by an encodingapparatus according to the present document.

FIG. 5 schematically shows an encoding apparatus for performing an imageencoding method according to this document.

FIG. 6 schematically shows an image decoding method by a decodingapparatus according to this document.

FIG. 7 schematically shows a decoding apparatus for performing an imagedecoding method according to this document.

FIG. 8 illustrates a structural diagram of a contents streaming systemto which the present disclosure is applied.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present disclosure may be modified in various forms, and specificembodiments thereof will be described and illustrated in the drawings.However, the embodiments are not intended for limiting the disclosure.The terms used in the following description are used to merely describespecific embodiments but are not intended to limit the disclosure. Anexpression of a singular number includes an expression of the pluralnumber, so long as it is clearly read differently. The terms such as“include” and “have” are intended to indicate that features, numbers,steps, operations, elements, components, or combinations thereof used inthe following description exist and it should be thus understood thatthe possibility of existence or addition of one or more differentfeatures, numbers, steps, operations, elements, components, orcombinations thereof is not excluded.

Meanwhile, elements in the drawings described in the disclosure areindependently drawn for the purpose of convenience for explanation ofdifferent specific functions, and do not mean that the elements areembodied by independent hardware or independent software. For example,two or more elements of the elements may be combined to form a singleelement, or one element may be partitioned into plural elements. Theembodiments in which the elements are combined and/or partitioned belongto the disclosure without departing from the concept of the disclosure.

Hereinafter, embodiments of the present disclosure will be described indetail with reference to the accompanying drawings. In addition, likereference numerals are used to indicate like elements throughout thedrawings, and the same descriptions on the like elements will beomitted.

FIG. 1 briefly illustrates an example of a video/image coding device towhich embodiments of the present disclosure are applicable.

Referring to FIG. 1, a video/image coding system may include a firstdevice (source device) and a second device (receiving device). Thesource device may deliver encoded video/image information or data in theform of a file or streaming to the receiving device via a digitalstorage medium or network.

The source device may include a video source, an encoding apparatus, anda transmitter. The receiving device may include a receiver, a decodingapparatus, and a renderer. The encoding apparatus may be called avideo/image encoding apparatus, and the decoding apparatus may be calleda video/image decoding apparatus. The transmitter may be included in theencoding apparatus. The receiver may be included in the decodingapparatus. The renderer may include a display, and the display may beconfigured as a separate device or an external component.

The video source may acquire video/image through a process of capturing,synthesizing, or generating the video/image. The video source mayinclude a video/image capture device and/or a video/image generatingdevice. The video/image capture device may include, for example, one ormore cameras, video/image archives including previously capturedvideo/images, and the like. The video/image generating device mayinclude, for example, computers, tablets and smartphones, and may(electronically) generate video/images. For example, a virtualvideo/image may be generated through a computer or the like. In thiscase, the video/image capturing process may be replaced by a process ofgenerating related data.

The encoding apparatus may encode input image/image. The encodingapparatus may perform a series of procedures such as prediction,transform, and quantization for compression and coding efficiency. Theencoded data (encoded video/image information) may be output in the formof a bitstream.

The transmitter may transmit the encoded image/image information or dataoutput in the form of a bitstream to the receiver of the receivingdevice through a digital storage medium or a network in the form of afile or streaming. The digital storage medium may include variousstorage mediums such as USB, SD, CD, DVD, Blu-ray, HDD, SSD, and thelike. The transmitter may include an element for generating a media filethrough a predetermined file format and may include an element fortransmission through a broadcast/communication network. The receiver mayreceive/extract the bitstream and transmit the received bitstream to thedecoding apparatus.

The decoding apparatus may decode the video/image by performing a seriesof procedures such as dequantization, inverse transform, and predictioncorresponding to the operation of the encoding apparatus.

The renderer may render the decoded video/image. The renderedvideo/image may be displayed through the display.

Present disclosure relates to video/image coding. For example, themethods/embodiments disclosed in the present disclosure may be appliedto a method disclosed in the versatile video coding (VVC), the EVC(essential video coding) standard, the AOMedia Video 1 (AV1) standard,the 2nd generation of audio video coding standard (AVS2), or the nextgeneration video/image coding standard (ex. H.267 or H.268, etc.).

Present disclosure presents various embodiments of video/image coding,and the embodiments may be performed in combination with each otherunless otherwise mentioned.

In the present disclosure, video may refer to a series of images overtime. Picture generally refers to a unit representing one image in aspecific time zone, and a subpicture/slice/tile is a unit constitutingpart of a picture in coding. The subpicture/slice/tile may include oneor more coding tree units (CTUs). One picture may consist of one or moresubpictures/slices/tiles. One picture may consist of one or more tilegroups. One tile group may include one or more tiles. A brick mayrepresent a rectangular region of CTU rows within a tile in a picture. Atile may be partitioned into multiple bricks, each of which consistingof one or more CTU rows within the tile. A tile that is not partitionedinto multiple bricks may be also referred to as a brick. A brick scan isa specific sequential ordering of CTUs partitioning a picture in whichthe CTUs are ordered consecutively in CTU raster scan in a brick, brickswithin a tile are ordered consecutively in a raster scan of the bricksof the tile, and tiles in a picture are ordered consecutively in araster scan of the tiles of the picture. In addition, a subpicture mayrepresent a rectangular region of one or more slices within a picture.That is, a subpicture contains one or more slices that collectivelycover a rectangular region of a picture. A tile is a rectangular regionof CTUs within a particular tile column and a particular tile row in apicture. The tile column is a rectangular region of CTUs having a heightequal to the height of the picture and a width specified by syntaxelements in the picture parameter set. The tile row is a rectangularregion of CTUs having a height specified by syntax elements in thepicture parameter set and a width equal to the width of the picture. Atile scan is a specific sequential ordering of CTUs partitioning apicture in which the CTUs are ordered consecutively in CTU raster scanin a tile whereas tiles in a picture are ordered consecutively in araster scan of the tiles of the picture. A slice includes an integernumber of bricks of a picture that may be exclusively contained in asingle NAL unit. A slice may consist of either a number of completetiles or only a consecutive sequence of complete bricks of one tile.Tile groups and slices may be used interchangeably in the presentdisclosure. For example, in the present disclosure, a tile group/tilegroup header may be called a slice/slice header.

A pixel or a pel may mean a smallest unit constituting one picture (orimage). Also, ‘sample’ may be used as a term corresponding to a pixel. Asample may generally represent a pixel or a value of a pixel, and mayrepresent only a pixel/pixel value of a luma component or only apixel/pixel value of a chroma component.

A unit may represent a basic unit of image processing. The unit mayinclude at least one of a specific region of the picture and informationrelated to the region. One unit may include one luma block and twochroma (ex. cb, cr) blocks. The unit may be used interchangeably withterms such as block or area in some cases. In a general case, an M×Nblock may include samples (or sample arrays) or a set (or array) oftransform coefficients of M columns and N rows.

In the present description, “A or B” may mean “only A”, “only B” or“both A and B”. In other words, in the present specification, “A or B”may be interpreted as “A and/or B”. For example, “A, B or C” hereinmeans “only A”, “only B”, “only C”, or “any and any combination of A, Band C”.

A slash (/) or a comma (comma) used in the present description may mean“and/or”. For example, “A/B” may mean “A and/or B”. Accordingly, “A/B”may mean “only A”, “only B”, or “both A and B”. For example, “A, B, C”may mean “A, B, or C”.

In the present description, “at least one of A and B” may mean “only A”,“only B”, or “both A and B”. In addition, in the present description,the expression “at least one of A or B” or “at least one of A and/or B”may be interpreted the same as “at least one of A and B”.

In addition, in the present description, “at least one of A, B and C”means “only A”, “only B”, “only C”, or “any combination of A, B and C”.Also, “at least one of A, B or C” or “at least one of A, B and/or C” maymean “at least one of A, B and C”.

In addition, parentheses used in the present description may mean “forexample”. Specifically, when “prediction (intra prediction)” isindicated, “intra prediction” may be proposed as an example of“prediction”. In other words, “prediction” in the present description isnot limited to “intra prediction”, and “intra prediction” may beproposed as an example of “prediction”. Also, even when “prediction (ie,intra prediction)” is indicated, “intra prediction” may be proposed asan example of “prediction”.

In the present description, technical features that are individuallydescribed within one drawing may be implemented individually or may beimplemented at the same time.

The following drawings were created to explain a specific example of thepresent description. Since the names of specific devices described inthe drawings or the names of specific signals/messages/fields arepresented by way of example, the technical features of the presentdescription are not limited to the specific names used in the followingdrawings.

FIG. 2 is a schematic diagram illustrating a configuration of avideo/image encoding apparatus to which the embodiment(s) of the presentdisclosure may be applied. Hereinafter, the video encoding apparatus mayinclude an image encoding apparatus.

Referring to FIG. 2, the encoding apparatus 200 includes an imagepartitioner 210, a predictor 220, a residual processor 230, and anentropy encoder 240, an adder 250, a filter 260, and a memory 270. Thepredictor 220 may include an inter predictor 221 and an intra predictor222. The residual processor 230 may include a transformer 232, aquantizer 233, a dequantizer 234, and an inverse transformer 235. Theresidual processor 230 may further include a subtractor 231. The adder250 may be called a reconstructor or a reconstructed block generator.The image partitioner 210, the predictor 220, the residual processor230, the entropy encoder 240, the adder 250, and the filter 260 may beconfigured by at least one hardware component (ex. An encoder chipset orprocessor) according to an embodiment. In addition, the memory 270 mayinclude a decoded picture buffer (DPB) or may be configured by a digitalstorage medium. The hardware component may further include the memory270 as an internal/external component.

The image partitioner 210 may partition an input image (or a picture ora frame) input to the encoding apparatus 200 into one or moreprocessors. For example, the processor may be called a coding unit (CU).In this case, the coding unit may be recursively partitioned accordingto a quad-tree binary-tree ternary-tree (QTBTTT) structure from a codingtree unit (CTU) or a largest coding unit (LCU). For example, one codingunit may be partitioned into a plurality of coding units of a deeperdepth based on a quad tree structure, a binary tree structure, and/or aternary structure. In this case, for example, the quad tree structuremay be applied first and the binary tree structure and/or ternarystructure may be applied later. Alternatively, the binary tree structuremay be applied first. The coding procedure according to the presentdisclosure may be performed based on the final coding unit that is nolonger partitioned. In this case, the largest coding unit may be used asthe final coding unit based on coding efficiency according to imagecharacteristics, or if necessary, the coding unit may be recursivelypartitioned into coding units of deeper depth and a coding unit havingan optimal size may be used as the final coding unit. Here, the codingprocedure may include a procedure of prediction, transform, andreconstruction, which will be described later. As another example, theprocessor may further include a prediction unit (PU) or a transform unit(TU). In this case, the prediction unit and the transform unit may besplit or partitioned from the aforementioned final coding unit. Theprediction unit may be a unit of sample prediction, and the transformunit may be a unit for deriving a transform coefficient and/or a unitfor deriving a residual signal from the transform coefficient.

The unit may be used interchangeably with terms such as block or area insome cases. In a general case, an M×N block may represent a set ofsamples or transform coefficients composed of M columns and N rows. Asample may generally represent a pixel or a value of a pixel, mayrepresent only a pixel/pixel value of a luma component or represent onlya pixel/pixel value of a chroma component. A sample may be used as aterm corresponding to one picture (or image) for a pixel or a pel.

In the encoding apparatus 200, a prediction signal (predicted block,prediction sample array) output from the inter predictor 221 or theintra predictor 222 is subtracted from an input image signal (originalblock, original sample array) to generate a residual signal residualblock, residual sample array), and the generated residual signal istransmitted to the transformer 232. In this case, as shown, a unit forsubtracting a prediction signal (predicted block, prediction samplearray) from the input image signal (original block, original samplearray) in the encoder 200 may be called a subtractor 231. The predictormay perform prediction on a block to be processed (hereinafter, referredto as a current block) and generate a predicted block includingprediction samples for the current block. The predictor may determinewhether intra prediction or inter prediction is applied on a currentblock or CU basis. As described later in the description of eachprediction mode, the predictor may generate various information relatedto prediction, such as prediction mode information, and transmit thegenerated information to the entropy encoder 240. The information on theprediction may be encoded in the entropy encoder 240 and output in theform of a bitstream.

The intra predictor 222 may predict the current block by referring tothe samples in the current picture. The referred samples may be locatedin the neighborhood of the current block or may be located apartaccording to the prediction mode. In the intra prediction, predictionmodes may include a plurality of non-directional modes and a pluralityof directional modes. The non-directional mode may include, for example,a DC mode and a planar mode. The directional mode may include, forexample, 33 directional prediction modes or 65 directional predictionmodes according to the degree of detail of the prediction direction.However, this is merely an example, more or less directional predictionmodes may be used depending on a setting. The intra predictor 222 maydetermine the prediction mode applied to the current block by using aprediction mode applied to a neighboring block.

The inter predictor 221 may derive a predicted block for the currentblock based on a reference block (reference sample array) specified by amotion vector on a reference picture. Here, in order to reduce theamount of motion information transmitted in the inter prediction mode,the motion information may be predicted in units of blocks, sub-blocks,or samples based on correlation of motion information between theneighboring block and the current block. The motion information mayinclude a motion vector and a reference picture index. The motioninformation may further include inter prediction direction (L0prediction, L1 prediction, Bi prediction, etc.) information. In the caseof inter prediction, the neighboring block may include a spatialneighboring block present in the current picture and a temporalneighboring block present in the reference picture. The referencepicture including the reference block and the reference pictureincluding the temporal neighboring block may be the same or different.The temporal neighboring block may be called a collocated referenceblock, a co-located CU (colCU), and the like, and the reference pictureincluding the temporal neighboring block may be called a collocatedpicture (colPic). For example, the inter predictor 221 may configure amotion information candidate list based on neighboring blocks andgenerate information indicating which candidate is used to derive amotion vector and/or a reference picture index of the current block.Inter prediction may be performed based on various prediction modes. Forexample, in the case of a skip mode and a merge mode, the interpredictor 221 may use motion information of the neighboring block asmotion information of the current block. In the skip mode, unlike themerge mode, the residual signal may not be transmitted. In the case ofthe motion vector prediction (MVP) mode, the motion vector of theneighboring block may be used as a motion vector predictor and themotion vector of the current block may be indicated by signaling amotion vector difference.

The predictor 220 may generate a prediction signal based on variousprediction methods described below. For example, the predictor may notonly apply intra prediction or inter prediction to predict one block butalso simultaneously apply both intra prediction and inter prediction.This may be called combined inter and intra prediction (CIIP). Inaddition, the predictor may be based on an intra block copy (IBC)prediction mode or a palette mode for prediction of a block. The IBCprediction mode or palette mode may be used for content image/videocoding of a game or the like, for example, screen content coding (SCC).The IBC basically performs prediction in the current picture but may beperformed similarly to inter prediction in that a reference block isderived in the current picture. That is, the IBC may use at least one ofthe inter prediction techniques described in the present disclosure. Thepalette mode may be considered as an example of intra coding or intraprediction. When the palette mode is applied, a sample value within apicture may be signaled based on information on the palette table andthe palette index.

The prediction signal generated by the predictor (including the interpredictor 221 and/or the intra predictor 222) may be used to generate areconstructed signal or to generate a residual signal. The transformer232 may generate transform coefficients by applying a transformtechnique to the residual signal. For example, the transform techniquemay include at least one of a discrete cosine transform (DCT), adiscrete sine transform (DST), a karhunen-loève transform (KLT), agraph-based transform (GBT), or a conditionally non-linear transform(CNT). Here, the GBT means transform obtained from a graph whenrelationship information between pixels is represented by the graph. TheCNT refers to transform generated based on a prediction signal generatedusing all previously reconstructed pixels. In addition, the transformprocess may be applied to square pixel blocks having the same size ormay be applied to blocks having a variable size rather than square.

The quantizer 233 may quantize the transform coefficients and transmitthem to the entropy encoder 240 and the entropy encoder 240 may encodethe quantized signal (information on the quantized transformcoefficients) and output a bitstream. The information on the quantizedtransform coefficients may be referred to as residual information. Thequantizer 233 may rearrange block type quantized transform coefficientsinto a one-dimensional vector form based on a coefficient scanning orderand generate information on the quantized transform coefficients basedon the quantized transform coefficients in the one-dimensional vectorform. Information on transform coefficients may be generated. Theentropy encoder 240 may perform various encoding methods such as, forexample, exponential Golomb, context-adaptive variable length coding(CAVLC), context-adaptive binary arithmetic coding (CABAC), and thelike. The entropy encoder 240 may encode information necessary forvideo/image reconstruction other than quantized transform coefficients(ex. values of syntax elements, etc.) together or separately. Encodedinformation (ex. encoded video/image information) may be transmitted orstored in units of NALs (network abstraction layer) in the form of abitstream. The video/image information may further include informationon various parameter sets such as an adaptation parameter set (APS), apicture parameter set (PPS), a sequence parameter set (SPS), or a videoparameter set (VPS). In addition, the video/image information mayfurther include general constraint information. In the presentdisclosure, information and/or syntax elements transmitted/signaled fromthe encoding apparatus to the decoding apparatus may be included invideo/picture information. The video/image information may be encodedthrough the above-described encoding procedure and included in thebitstream. The bitstream may be transmitted over a network or may bestored in a digital storage medium. The network may include abroadcasting network and/or a communication network, and the digitalstorage medium may include various storage media such as USB, SD, CD,DVD, Blu-ray, HDD, SSD, and the like. A transmitter (not shown)transmitting a signal output from the entropy encoder 240 and/or astorage unit (not shown) storing the signal may be included asinternal/external element of the encoding apparatus 200, andalternatively, the transmitter may be included in the entropy encoder240.

The quantized transform coefficients output from the quantizer 233 maybe used to generate a prediction signal. For example, the residualsignal (residual block or residual samples) may be reconstructed byapplying dequantization and inverse transform to the quantized transformcoefficients through the dequantizer 234 and the inverse transformer235. The adder 250 adds the reconstructed residual signal to theprediction signal output from the inter predictor 221 or the intrapredictor 222 to generate a reconstructed signal (reconstructed picture,reconstructed block, reconstructed sample array). If there is noresidual for the block to be processed, such as a case where the skipmode is applied, the predicted block may be used as the reconstructedblock. The adder 250 may be called a reconstructor or a reconstructedblock generator. The generated reconstructed signal may be used forintra prediction of a next block to be processed in the current pictureand may be used for inter prediction of a next picture through filteringas described below.

Meanwhile, luma mapping with chroma scaling (LMCS) may be applied duringpicture encoding and/or reconstruction.

The filter 260 may improve subjective/objective image quality byapplying filtering to the reconstructed signal. For example, the filter260 may generate a modified reconstructed picture by applying variousfiltering methods to the reconstructed picture and store the modifiedreconstructed picture in the memory 270, specifically, a DPB of thememory 270. The various filtering methods may include, for example,deblocking filtering, a sample adaptive offset, an adaptive loop filter,a bilateral filter, and the like. The filter 260 may generate variousinformation related to the filtering and transmit the generatedinformation to the entropy encoder 240 as described later in thedescription of each filtering method. The information related to thefiltering may be encoded by the entropy encoder 240 and output in theform of a bitstream.

The modified reconstructed picture transmitted to the memory 270 may beused as the reference picture in the inter predictor 221. When the interprediction is applied through the encoding apparatus, predictionmismatch between the encoding apparatus 200 and the decoding apparatus300 may be avoided and encoding efficiency may be improved.

The DPB of the memory 270 DPB may store the modified reconstructedpicture for use as a reference picture in the inter predictor 221. Thememory 270 may store the motion information of the block from which themotion information in the current picture is derived (or encoded) and/orthe motion information of the blocks in the picture that have alreadybeen reconstructed. The stored motion information may be transmitted tothe inter predictor 221 and used as the motion information of thespatial neighboring block or the motion information of the temporalneighboring block. The memory 270 may store reconstructed samples ofreconstructed blocks in the current picture and may transfer thereconstructed samples to the intra predictor 222.

FIG. 3 is a schematic diagram illustrating a configuration of avideo/image decoding apparatus to which the embodiment(s) of the presentdisclosure may be applied.

Referring to FIG. 3, the decoding apparatus 300 may include an entropydecoder 310, a residual processor 320, a predictor 330, an adder 340, afilter 350, a memory 360. The predictor 330 may include an interpredictor 331 and an intra predictor 332. The residual processor 320 mayinclude a dequantizer 321 and an inverse transformer 322. The entropydecoder 310, the residual processor 320, the predictor 330, the adder340, and the filter 350 may be configured by a hardware component (ex. Adecoder chipset or a processor) according to an embodiment. In addition,the memory 360 may include a decoded picture buffer (DPB) or may beconfigured by a digital storage medium. The hardware component mayfurther include the memory 360 as an internal/external component.

When a bitstream including video/image information is input, thedecoding apparatus 300 may reconstruct an image corresponding to aprocess in which the video/image information is processed in theencoding apparatus of FIG. 2. For example, the decoding apparatus 300may derive units/blocks based on block partition related informationobtained from the bitstream. The decoding apparatus 300 may performdecoding using a processor applied in the encoding apparatus. Thus, theprocessor of decoding may be a coding unit, for example, and the codingunit may be partitioned according to a quad tree structure, binary treestructure and/or ternary tree structure from the coding tree unit or thelargest coding unit. One or more transform units may be derived from thecoding unit. The reconstructed image signal decoded and output throughthe decoding apparatus 300 may be reproduced through a reproducingapparatus.

The decoding apparatus 300 may receive a signal output from the encodingapparatus of FIG. 2 in the form of a bitstream, and the received signalmay be decoded through the entropy decoder 310. For example, the entropydecoder 310 may parse the bitstream to derive information (ex.video/image information) necessary for image reconstruction (or picturereconstruction). The video/image information may further includeinformation on various parameter sets such as an adaptation parameterset (APS), a picture parameter set (PPS), a sequence parameter set(SPS), or a video parameter set (VPS). In addition, the video/imageinformation may further include general constraint information. Thedecoding apparatus may further decode picture based on the informationon the parameter set and/or the general constraint information.Signaled/received information and/or syntax elements described later inthe present disclosure may be decoded may decode the decoding procedureand obtained from the bitstream. For example, the entropy decoder 310decodes the information in the bitstream based on a coding method suchas exponential Golomb coding, CAVLC, or CABAC, and output syntaxelements required for image reconstruction and quantized values oftransform coefficients for residual. More specifically, the CABACentropy decoding method may receive a bin corresponding to each syntaxelement in the bitstream, determine a context model using a decodingtarget syntax element information, decoding information of a decodingtarget block or information of a symbol/bin decoded in a previous stage,and perform an arithmetic decoding on the bin by predicting aprobability of occurrence of a bin according to the determined contextmodel, and generate a symbol corresponding to the value of each syntaxelement. In this case, the CABAC entropy decoding method may update thecontext model by using the information of the decoded symbol/bin for acontext model of a next symbol/bin after determining the context model.The information related to the prediction among the information decodedby the entropy decoder 310 may be provided to the predictor (the interpredictor 332 and the intra predictor 331), and the residual value onwhich the entropy decoding was performed in the entropy decoder 310,that is, the quantized transform coefficients and related parameterinformation, may be input to the residual processor 320. The residualprocessor 320 may derive the residual signal (the residual block, theresidual samples, the residual sample array). In addition, informationon filtering among information decoded by the entropy decoder 310 may beprovided to the filter 350. Meanwhile, a receiver (not shown) forreceiving a signal output from the encoding apparatus may be furtherconfigured as an internal/external element of the decoding apparatus300, or the receiver may be a component of the entropy decoder 310.Meanwhile, the decoding apparatus according to the present disclosuremay be referred to as a video/image/picture decoding apparatus, and thedecoding apparatus may be classified into an information decoder(video/image/picture information decoder) and a sample decoder(video/image/picture sample decoder). The information decoder mayinclude the entropy decoder 310, and the sample decoder may include atleast one of the dequantizer 321, the inverse transformer 322, the adder340, the filter 350, the memory 360, the inter predictor 332, and theintra predictor 331.

The dequantizer 321 may dequantize the quantized transform coefficientsand output the transform coefficients. The dequantizer 321 may rearrangethe quantized transform coefficients in the form of a two-dimensionalblock form. In this case, the rearrangement may be performed based onthe coefficient scanning order performed in the encoding apparatus. Thedequantizer 321 may perform dequantization on the quantized transformcoefficients by using a quantization parameter (ex. quantization stepsize information) and obtain transform coefficients.

The inverse transformer 322 inversely transforms the transformcoefficients to obtain a residual signal (residual block, residualsample array).

The predictor may perform prediction on the current block and generate apredicted block including prediction samples for the current block. Thepredictor may determine whether intra prediction or inter prediction isapplied to the current block based on the information on the predictionoutput from the entropy decoder 310 and may determine a specificintra/inter prediction mode.

The predictor 320 may generate a prediction signal based on variousprediction methods described below. For example, the predictor may notonly apply intra prediction or inter prediction to predict one block butalso simultaneously apply intra prediction and inter prediction. Thismay be called combined inter and intra prediction (CIIP). In addition,the predictor may be based on an intra block copy (IBC) prediction modeor a palette mode for prediction of a block. The IBC prediction mode orpalette mode may be used for content image/video coding of a game or thelike, for example, screen content coding (SCC). The IBC basicallyperforms prediction in the current picture but may be performedsimilarly to inter prediction in that a reference block is derived inthe current picture. That is, the IBC may use at least one of the interprediction techniques described in the present disclosure. The palettemode may be considered as an example of intra coding or intraprediction. When the palette mode is applied, a sample value within apicture may be signaled based on information on the palette table andthe palette index.

The intra predictor 331 may predict the current block by referring tothe samples in the current picture. The referred samples may be locatedin the neighborhood of the current block or may be located apartaccording to the prediction mode. In the intra prediction, predictionmodes may include a plurality of non-directional modes and a pluralityof directional modes. The intra predictor 331 may determine theprediction mode applied to the current block by using a prediction modeapplied to a neighboring block.

The inter predictor 332 may derive a predicted block for the currentblock based on a reference block (reference sample array) specified by amotion vector on a reference picture. In this case, in order to reducethe amount of motion information transmitted in the inter predictionmode, motion information may be predicted in units of blocks,sub-blocks, or samples based on correlation of motion informationbetween the neighboring block and the current block. The motioninformation may include a motion vector and a reference picture index.The motion information may further include inter prediction direction(L0 prediction, L1 prediction, Bi prediction, etc.) information. In thecase of inter prediction, the neighboring block may include a spatialneighboring block present in the current picture and a temporalneighboring block present in the reference picture. For example, theinter predictor 332 may configure a motion information candidate listbased on neighboring blocks and derive a motion vector of the currentblock and/or a reference picture index based on the received candidateselection information. Inter prediction may be performed based onvarious prediction modes, and the information on the prediction mayinclude information indicating a mode of inter prediction for thecurrent block.

The adder 340 may generate a reconstructed signal (reconstructedpicture, reconstructed block, reconstructed sample array) by adding theobtained residual signal to the prediction signal (predicted block,predicted sample array) output from the predictor (including the interpredictor 332 and/or the intra predictor 331). If there is no residualfor the block to be processed, such as when the skip mode is applied,the predicted block may be used as the reconstructed block.

The adder 340 may be called reconstructor or a reconstructed blockgenerator. The generated reconstructed signal may be used for intraprediction of a next block to be processed in the current picture, maybe output through filtering as described below, or may be used for interprediction of a next picture.

Meanwhile, luma mapping with chroma scaling (LMCS) may be applied in thepicture decoding process.

The filter 350 may improve subjective/objective image quality byapplying filtering to the reconstructed signal. For example, the filter350 may generate a modified reconstructed picture by applying variousfiltering methods to the reconstructed picture and store the modifiedreconstructed picture in the memory 360, specifically, a DPB of thememory 360. The various filtering methods may include, for example,deblocking filtering, a sample adaptive offset, an adaptive loop filter,a bilateral filter, and the like.

The (modified) reconstructed picture stored in the DPB of the memory 360may be used as a reference picture in the inter predictor 332. Thememory 360 may store the motion information of the block from which themotion information in the current picture is derived (or decoded) and/orthe motion information of the blocks in the picture that have alreadybeen reconstructed. The stored motion information may be transmitted tothe inter predictor 260 so as to be utilized as the motion informationof the spatial neighboring block or the motion information of thetemporal neighboring block. The memory 360 may store reconstructedsamples of reconstructed blocks in the current picture and transfer thereconstructed samples to the intra predictor 331.

In the present disclosure, the embodiments described in the filter 260,the inter predictor 221, and the intra predictor 222 of the encodingapparatus 200 may be the same as or respectively applied to correspondto the filter 350, the inter predictor 332, and the intra predictor 331of the decoding apparatus 300. The same may also apply to the unit 332and the intra predictor 331.

In the present disclosure, at least one of quantization/inversequantization and/or transform/inverse transform may be omitted. When thequantization/inverse quantization is omitted, the quantized transformcoefficients may be called transform coefficients. When thetransform/inverse transform is omitted, the transform coefficients maybe called coefficients or residual coefficients, or may still be calledtransform coefficients for uniformity of expression.

In the present disclosure, a quantized transform coefficient and atransform coefficient may be referred to as a transform coefficient anda scaled transform coefficient, respectively. In this case, the residualinformation may include information on transform coefficient(s), and theinformation on the transform coefficient(s) may be signaled throughresidual coding syntax. Transform coefficients may be derived based onthe residual information (or the information on the transformcoefficient(s)), and scaled transform coefficients may be derived byinverse transforming (scaling) on the transform coefficients. Residualsamples may be derived based on the inverse transforming (transforming)on the scaled transform coefficients. This may be applied/expressed inother parts of the present disclosure as well.

Meanwhile, as described above, the quantizer of the encoding apparatusmay derive quantized transform coefficients by applying quantization totransform coefficients. The dequantizer of the encoding apparatus or thedequantizer of the decoding apparatus may derive transform coefficientsby applying dequantization to the quantized transform coefficients.

In general, in video/image coding, a quantization ratio may be changed,and a compression rate may be adjusted using the changed quantizationratio. In an implementation aspect, a quantization parameter (QP) may beused instead of directly using the quantization ratio by consideringcomplexity. For example, quantization parameters having integer valuesof 0 to 63 may be used, and each quantization parameter value maycorrespond to an actual quantization ratio. Furthermore, for example, aquantization parameter QP_(Y) for a luma component and a quantizationparameter QP_(C) for a chroma component may be different configured.

In a quantization process, a transform coefficient C may be an input, aquantization ratio (Q_(step)) may be divided, and a quantized transformcoefficient C′ may be obtained based on the quantization ratio. In thiscase, the quantization ratio may be produced in an integer form bymultiplying the quantization ratio by a scale by considering calculationcomplexity, and a shift operation may be performed by a valuecorresponding to a scale value. A quantization scale may be derivedbased on the product of the quantization ratio and the scale value. Thatis, the quantization scale may be derived based on the QP. For example,the quantization scale may be applied to the transform coefficient C′,and a quantized transform coefficient C′ may be derived based on aresult of the application.

A dequantization process is a reverse process of the quantizationprocess. In this process, a quantized transform coefficient C may bemultiplied by a quantization ratio (Q_(step)), and a reconstructedtransform coefficient C may be obtained based on the result of themultiplication. In this case, a level scale may be derived based on aquantization parameter, the level scale may be applied to the quantizedtransform coefficient C′, and a reconstructed transform coefficient Cmay be derived. The reconstructed transform coefficient C may have somedifference from the first transform coefficient C due to a loss in thetransform and/or quantization process. Accordingly, dequantization isperformed in the encoding apparatus as in the decoding apparatus.

Meanwhile, an adaptive frequency weighting quantization technology foradjusting quantization strength depending on a frequency may be applied.The adaptive frequency weighting quantization technology is a method ofdifferently applying quantization strength for each frequency. In theadaptive frequency weighting quantization, quantization strength foreach frequency may be differently applied using a predefinedquantization scaling matrix. That is, the aforementionedquantization/dequantization process may be performed based on thequantization scaling matrix. For example, in order to generate the sizeof a current block and/or a residual signal of the current block, adifferent quantization scaling matrix may be used depending on whether aprediction mode applied to the current block is inter prediction orintra prediction. The quantization scaling matrix may be called aquantization matrix or a scaling matrix. The quantization scaling matrixmay be pre-defined. Furthermore, for frequency adaptive scaling,quantization scale information for each frequency for the quantizationscaling matrix may be constructed/encoded in the encoding apparatus andsignaled to the decoding apparatus. The quantization scale informationfor each frequency may be called quantization scaling information. Thequantization scale information for each frequency may include scalinglist data (scaling_list_data). A (modified) quantization scaling matrixmay be derived based on the scaling list data. Furthermore, thequantization scale information for each frequency may include presentflag information representing whether the scaling list data is present.Alternatively, if the scaling list data is signaled in a higher level(e.g., SPS), information representing whether the scaling list data ismodified in a lower level (e.g., a PPS or a tile group header, etc.) ofa higher level, etc. may be further included.

As in the aforementioned contents, quantization/dequantization may beapplied to a luma component and a chroma component based on aquantization parameter.

Quantization parameters for a coding unit may be determined based oninformation signaled in a picture and/or a slice level. For example, thequantization parameters may be derived as in contents described later.

For example, information related to the derivation of quantizationparameters may be signaled as in the following table through a sequenceparameter set (SPS).

TABLE 1 Descriptor seq_parameter_set_rbsp( ) { sps_decoding_parameter_set_id u(4) ...  bit_depth_luma_minus8 ue(v) bit_depth_chroma_minus8 ue(v) ...  rbsp_trailing_bits( ) }

Semantics for syntax elements in Table 1 may be the same as thefollowing table.

TABLE 2 bit_depth_luma_minus8 specifics the bit depth of the samples ofthe luma array BitDepth_(Y) and the value of the luma quantizationparameter range offset QpBdOffset_(Y) as follows:  BitDepth_(Y) = 8 +bit_depth_luma_minus8  QpBdOffset_(Y) = 6 * bit_depth_luma_minus8bit_depth_luma_minus8 shall be in the range of 0 to 8, inclusive.bit_depth_chroma_minus8_specifics the bit depth of the samples of thechroma arrays BitDepth_(C) and the value of the chroma quantizationparameter range offset QpBdOffset_(C) as follows:  BitDepth_(C) = 8 + bit_depth_chroma_minus8  QpBdOffset_(C) = 6 * bit_depth_chroma_minus8bit_depth_chroma_minus8 shall be in the range of 0 to 8, inclusive.

For example, the syntax element bit_depth_luma_minus8 may representBitDepth_(Y), that is, the bit depth of samples of a luma array, andQpBdOffset_(Y) that is a luma quantization parameter range offset. Thatis, for example, the BitDepth_(Y) and the QpBdOffset_(Y) may be derivedbased on the syntax element bit_depth_luma_minus8. For example, theBitDepth_(Y) may be derived as a value obtained by adding 8 to a valueof the syntax element bit_depth_luma_minus8. The QpBdOffset_(Y) may bederived as a value obtained by multiplying a value of the syntax elementbit_depth_luma_minus8 by 6. Furthermore, the bit_depth_luma_minus8 maybe in a range of 0 to 8.

Furthermore, for example, the syntax element bit_depth_chroma_minus8 mayrepresent BitDepth_(C), that is, the bit depth of samples of a chromaarray, and QpBdOffset_(C), that is, a chroma quantization parameterrange offset. That is, for example, the BitDepth_(C) and theQpBdOffset_(C) may be derived based on the syntax elementbit_depth_chroma_minus8. For example, the BitDepth_(C) may be derived asa value obtained by adding 8 to a value of the syntax elementbit_depth_chroma_minus8. The QpBdOffset_(C) may be derived as a valueobtained by multiplying a value of the syntax elementbit_depth_chroma_minus8 by 6. Furthermore, the bit_depth_chroma_minus8may be in a range of 0 to 8.

Furthermore, information related to the derivation of quantizationparameters may be signaled as in the following table, for example,through a picture parameter set (PPS). The information may include achroma Cb offset, a chroma Cr offset, a joint chroma offset, and aninitial quantization parameter. That is, the information may includesyntax elements for a chroma Cb offset, a chroma Cr offset, a jointchroma offset, and an initial quantization parameter.

TABLE 3 Descriptor pic_parameter_set_rbsp( ) {  pps_pic_parameter_set_idue(v) ...  init_qp_minus26 se(v)  transform_skip_enabled_flag u(1) pps_cb_qp_offset se(v)  pps_cr_qp_offset se(v) pps_joint_cbcr_qp_offset se(v) pps_slice_chroma_qp_offsets_present_flag u(1) ...  rbsp_trailing_bits() }

Semantics for syntax elements in Table 3 may be the same as thefollowing table.

TABLE 4 init_qp_minus26 plus 26 specifies the initial value ofSliceQp_(Y) for each slice referring to the PPS. The initial value ofSliceQp_(Y) is modified at the slice layer when a non-zero value ofslice_qp_delta is decoded. The value of init_qp_minus26 shall be in therange of −( 26 + QpBdOffset_(Y) ) to +37, inclusive. pps_cb_qp_offsetand pps_cr_qp_offset specify the offsets to the luma quantizationparameter Qp′_(Y) used for deriving Qp′_(Cb) and Qp′_(Cr), respectively.The values of pps_cb_qp_offset and pps_cr_qp_offset shall be in therange of −12 to +12, inclusive. When ChromaArrayType is equal to 0,pps_cb_qp_offset and pps_cr_qp_offset are not used in the decodingprocess and decoders shall ignore their value. pps_joint_cbcr_qp_offsetspecifies the offset to the luma quantization parameter Qp′_(Y) used forderiving Qp′_(CbCr). The value of pps_joint_cbcr_qp_offset shall be inthe range of −12 to +12, inclusive. When ChromaArrayType is equal to 0,pps_joint_cbcr_qp_offset is not used in the decoding process anddecoders shall ignore its value.pps_slice_chroma_qp_offsets_present_flag equal to 1 indicates that theslice_cb_qp_offset and slice_cr_qp_offset syntax elements are present inthe associated slice headers. pps_slice_chroma_qp_offsets_present_flagequal to 0 indicates that these syntax elements are not present in theassociated slice headers. When ChromaArrayType is equal to 0,pps_slice_chroma_qp_offsets_present_flag shall be equal to 0.

For example, a value obtained by adding 26 to the syntax elementinit_qp_minus26 may represent an initial value of SliceQp_(Y) for eachslice that refers to a PPS. If a non-zero value of slice_qp_delta isdecoded, an initial value of the SliceQpY may be modified in a slicelayer. The init_qp_minus26 0 may be in a range of −(26+QpBdOffset_(Y))to +37.

Furthermore, for example, syntax elements pps_cb_qp_offset andpps_cr_qp_offset may represent offsets for a luma quantization parameterQp′_(Y) used to derive Qp′_(Cb) and Qp′_(Cr), respectively. Thepps_cb_qp_offset and pps_cr_qp_offset may be in a range of −12 to +12.Furthermore, when ChromaArrayType is 0, in a decoding process,pps_cb_qp_offset and pps_cr_qp_offset may not be used, and the decodingapparatus may ignore values of the syntax elements.

Furthermore, for example, the syntax element pps_joint_cbcr_qp_offsetmay represent an offset for a luma quantization parameter Qp′_(Y) usedto derive Qp′_(CbCr). The pps_joint_cbcr_qp_offset may be in a range of−12 to +12. Furthermore, when ChromaArrayType is 0, in a decodingprocess, pps_joint_cbcr_qp_offset may not be used, and the decodingapparatus may ignore a value of the syntax element.

Furthermore, for example, the syntax elementpps_slice_chroma_qp_offsets_present_flag may represent whether syntaxelements slice_cb_qp_offset and slice_cr_qp_offset are present in sliceheaders associated with the syntax elements slice_cb_qp_offset andslice_cr_qp_offset. For example,pps_slice_chroma_qp_offsets_present_flag having a value of 1 mayrepresent that the syntax elements slice_cb_qp_offset andslice_cr_qp_offset are present in slice headers associated with thesyntax elements slice_cb_qp_offset and slice_cr_qp_offset. Furthermore,for example, pps_slice_chroma_qp_offsets_present_flag having a value of0 may represent that the syntax elements slice_cb_qp_offset andslice_cr_qp_offset are not present in slice headers associated with thesyntax elements slice_cb_qp_offset and slice_cr_qp_offset. Furthermore,when ChromaArrayType is 0, in a decoding process,pps_slice_chroma_qp_offsets_present_flag may be the same as 0.

As in the aforementioned contents, syntax elements parsed in the PPS maybe init_qp_minus26, pps_cb_qp_offset_pps_cr_qp_offset,pps_joint_cbcr_qp_offset, and pps_slice_chroma_qp_offsets_present_flag.A syntax element init_qp_minus26 may represent an initial value ofSliceQp_(Y) for each slice that refers to a PPS. Furthermore, syntaxelements pps_cb_qp_offset, pps_cr_qp_offset, andpps_joint_cbcr_qp_offset may represent offsets for a luma quantizationparameter Qp′_(Y). Furthermore, the syntax elementpps_slice_chroma_qp_offsets_present_flag may represent whether an offsetparameter is present in a slice header.

Furthermore, information related to the derivation of quantizationparameters may be signaled as in the following table through a sliceheader, for example.

TABLE 5 Descriptor slice_header( ) {  slice_pic_parameter_set_id ue(v)...  slice_qp_delta se(v)  if( pps_slice_chroma_qp_offsets_present_flag) {   slice_cb_qp_offset se(v)   slice_cr_qp_offset se(v)  slice_joint_cbcr_qp_offset se(v)  } ...  byte_alignment( ) }

Semantics for syntax elements in Table 5 may be the same as thefollowing table.

TABLE 6 slice_qp_delta specifies the initial value of Qp_(Y) to be usedfor the coding blocks in the slice until modified by the value ofCuQpDeltaVal in the coding unit layer. The initial value of the Qp_(Y)quantization parameter for the slice, SliceQp_(Y), is derived asfollows:  SliceQp_(Y) = 26 + init_qp_minus26 + slice_qp_delta The valueof SliceQp_(Y) shall be in the range of −QpBdOffset_(Y) to +63,inclusive. slice_cb_qp_offset specifies a difference to be added to thevalue of pps_cb_qp_offset when determining the value of the Qp′_(Cb)quantization parameter. The value of slice_cb_qp_offset shall be in therange of −12 to +12, inclusive. When slice_cb_qp_offset is not present,it is inferred to be equal to 0. The value of pps_cb_qp_offset +slice_cb_qp_offset shall be in the range of −12 to +12, inclusive.slice_cr_qp_offset specifies a difference to be added to the value ofpps_cr_qp_offset when determining the value of the Qp′_(Cr) quantizationparameter. The value of slice_cr_qp_offset shall be in the range of −12to +12, inclusive. When slice_cr_qp_offset is not present, it isinferred to be equal to 0. The value of pps_cr_qp_offset +slice_cr_qp_offset shall be in the range of −12 to +12, inclusive.slice_joint_cbcr_qp_offset specifies a difference to be added to thevalue of pps_joint_cbcr_qp_offset when determining the value of theQp′_(CbCr). The value of slice_joint_cbcr_qp_offset shall be in therange of −12 to +12, inclusive. When slice_joint_cbcr_qp_offset is notpresent, it is inferred to be equal to 0. The value ofpps_joint_cbcr_qp_offset + slice_joint_cbcr_qp_offset shall be in therange of −12 to +12, inclusive. The derivation process for the luma andchroma quantization parameters begin with the inputs to the processbeing a luma location, variables specifying the width and height of thecurrent coding block and a variable specifying whether it is a single ora dual tree. The luma, chroma and the joint chroma quantizationparameters are dentoted as follows: Qp′_(Y) Qp′_(Cb) Qp′_(Cr) andQp′_(CbCr). cu_qp_delta_sign_flag specifies the sign of CuQpDeltaVal asfollows: - If cu_qp_delta_sign_flag is equal to 0, the correspondingCuQpDeltaVal has a positive value. - Otherwise (cu_qp_delta_sign_flag isequal to 1), the corresponding CuQpDeltaVal has a negative value. Whencu_qp_delta_sign_flag is not present, it is inferred to be equal to 0.When cu_qp_delta_abs is present, the variables IsCuQpDeltaCoded andCuQpDeltaVal are derived as follows:  IsCuQpDeltaCoded = 1  CuQpDeltaVal= cu_qp_delta_abs * ( 1 − 2 * cu_qp_delta_sign_flag ) The value ofCuQpDeltaVal shall be in the range of −( 32 + QpBdOffsetY / 2 ) to +(31 + QpBdOffsetY / 2 ), inclusive.

For example, slice_qp_delta may represent an initial value of Qp_(Y) tobe used in a coding block within a slice until it is modified by a valueof CuQpDeltaVal in a coding unit layer. For example, an initial value ofQp_(Y) for a slice, SliceQp_(Y), may be derived as26+init_qp_minus26+slice_qp_delta. A value of SliceQp_(Y) may be in arange of −QpBdOffset_(Y) to +63.

Furthermore, for example, slice_cb_qp_offset may represent a differenceto be added to a value of pps_cb_qp_offset when a value of thequantization parameter Qp′_(Cb) is determined. A value ofslice_cb_qp_offset may be in a range of −12 to +12. Furthermore, forexample, if slice_cb_qp_offset is not present, the slice_cb_qp_offsetmay be inferred as 0. A value of pps_cb_qp_offset+slice_cb_qp_offset maybe in a range of 12 to +12.

Furthermore, for example, slice_cr_qp_offset may represent a differenceto be added to a value of pps_cr_qp_offset when a value of aquantization parameter Qp′_(Cr) is determined. A value ofslice_cr_qp_offset may be in a range of −12 to +12. Furthermore, forexample, if slice_cr_qp_offset is not present, the slice_cr_qp_offsetmay be inferred as 0. A value of pps_cr_qp_offset+slice_cr_qp_offset maybe in a range of 12 to +12.

Furthermore, for example, slice_cbcr_qp_offset may represent adifference to be added to a value of pps_cbcr_qp_offset when a value ofa quantization parameter Qp′_(CbCr) is determined. A value ofslice_cbcr_qp_offset may be in a range of −12 to +12. Furthermore, forexample, if slice_cbcr_qp_offset is not present, theslice_cbcr_qp_offset may be inferred as 0. A value ofpps_cbcr_qp_offset+slice_cbcr_qp_offset may be in a range of 12 to +12.

A derivation process for luma and chroma quantization parameters may bestarted based on the fact that an input for the process is a lumalocation, a parameter to designate the width and height of a currentcoding block, and a parameter to designate a single tree or a dual tree.Meanwhile, as in the aforementioned contents, a luma quantizationparameter, a chroma quantization parameter and a joint chromaquantization parameter may be represented as Qp′_(Y), Qp′_(Cb), Qp′_(Cr)and Qp′_(CbCr).

Meanwhile, for example, the syntax element cu_qp_delta_sign_flagrepresenting a sign of CuQpDeltaVal may be parsed. For example, thecu_qp_delta_sign_flag may represent the sign of CuQpDeltaVal as follows.

For example, when the cu_qp_delta_sign_flag is 0, CuQpDeltaValcorresponding to the cu_qp_delta_sign_flag may have a positive value.Alternatively, for example, when the cu_qp_delta_sign_flag is 1,CuQpDeltaVal corresponding to the cu_qp_delta_sign_flag may have anegative value. Furthermore, if the cu_qp_delta_sign_flag is notpresent, the cu_qp_delta_sign_flag may be inferred as 0.

Furthermore, for example, if cu_qp_delta_abs is present, a parameterIsCuQpDeltaCoded may be derived as 1. A parameter CuQpDeltaVal may bederived as cu_qp_delta_abs*(1-2*cu_qp_delta_sign_flag). The CuQpDeltaValmay be in a range of −(32+QpBdOffsetY/2) to +(31+QpBdOffsetY/2).

Thereafter, for example, the luma quantization parameter Qp′_(Y) may bederived as in the following equation.

Qp _(Y)=((qP _(Y_PRED)+CuQpDeltaVal+64+2*QpBdOffset_(Y))%(64+QpBdOffset_(Y)))−QpBdOffset_(Y)  [Equation1]

Furthermore, if ChromaArrayType is not 0 and, treeType is SINGLE TREE orDUAL TREE CHROMA, the following may be applied.

-   -   When treeType is equal to DUAL TREE CHROMA, a parameter Qp_(Y)        may be set identically with a luma quantization parameter Qp_(Y)        of a luma coding unit including a luma location (xCb+cbWidth/2,        yCb+cbHeight/2).    -   Parameters qP_(Cb), qP_(Cr) and qP_(CbCr) may be derived as        follows.

qPi _(Cb)=Clip3(−QpBdOffset_(C),69,Qp _(Y)+pps_cb_qp_offset+slice_cb_qp_offset)

qPi _(Cr)=Clip3(−QpBdOffset_(C),69,Qp _(Y)+pps_cr_qp_offset+slice_cr_qp_offset)

qPi _(CbCr)=Clip3(−QpBdOffset_(C),69,Qp _(Y)+pps_joint_cbcr_qp_offset+slice_joint_cbcr_qp_offset)  [Equation 2]

For example, when ChromaArrayType is 1, parameters qP_(Cb), qP_(Cr) andqP_(CbCr) may be set identically with a QpC value designated in Table 7based on the same indices qPi as qPi_(Cb), qPi_(Cr) and qPi_(CbCr),respectively.

TABLE 7 qPi <30 30 31 32 33 34 35 36 37 38 39 40 41 42 43 >43 QP_(C)=qPi 29 30 31 32 33 33 34 34 35 35 36 36 37 37 =qPi − 6

Alternatively, when ChromaArrayType is not 1, the parameters qP_(Cb),qP_(Cr), and qP_(CbCr) may be set identically with Min (qPi, 63) basedon the same indices qPi as qPi_(Cb), qPi_(Cr), and qPi_(Cbc),respectively.

-   -   Chroma quantization parameters for a Cb component and a Cr        component, Qp′_(Cb) and Qp′_(Cr), and a chroma quantization        parameter Qp′_(CbCr) for joint Cb-Cr coding may be derived as        follows.

Qp′ _(Cb) =qP _(Cb) +QpBdOffset_(C)

Qp′ _(Cr) =qP _(Cr) +QpBdOffset_(C)

QP′ _(CbCr) =qP _(CbCr) +QpBdOffset_(C)  [Equation 3]

Meanwhile, this document proposes schemes for improving codingefficiency in a quantization/dequantization process.

In an embodiment, this document proposes a method of defining and usinga user defined chroma quantization mapping table, not a method ofobtaining a chroma quantization parameter value from a luma quantizationparameter value through a chroma quantization mapping table predefinedin the existing VVC Draft5 v.7 when ChromaArrayType is not 0 (e.g., whenChromaArrayType is 1). In the VVC specification text (e.g., VVC Draft5v.7), when qPi (a luma quantization parameter value) is given, Qpc(chroma quantization parameter value) is derived through the predefinedchroma quantization table (e.g., Table 7), but this document proposes amethod of deriving Qpc from qPi based on a chroma quantization mappingtable newly defined by a user. According to an embodiment of thisdocument, there is proposed a method in which a Qpc value may be derivedthrough a function relation of a qPi value, a function may be signaledas a syntax, such as an APS, an SPS or a PPS, through a user definedfunctionality method, which includes the function relation transmitvalues of predefined syntax elements, and a user defines chromaquantization table mapping based on the transmitted values. For example,since a Qpc value can be derived through the function relation of theqPi value, if syntax element values representing a correspondingfunction are transmitted, a user defined chroma quantization mappingtable may be derived in a form, such as Table 7.

In an embodiment, there is proposed a scheme for signaling informationabout syntax elements (Qpc_data) representing chroma quantizationmapping-related function as in the following table to be described laterin an adaptation parameter set (APS).

TABLE 8 Descriptor adaptation_parameter_set_rbsp( ) { adaptation_parameter_set_id u(5)  aps_params_type u(3)  if(aps_params_type = = ALF_APS )   alf_data(adaptation_parameter_set_id ) else if ( aps_params_type = = LMCS_APS )   lmcs_data( )  else if (aps_params_type = = Qpc_APS ) // 2   Qpc_data( )  aps_extension_flagu(1)  if( aps_extension_flag )   while( more_rbsp_data( ) )   aps_extension_data_flag u(1)  rbsp_trailing_bits( ) }

Referring to Table 8, if the aps_params_type represents Qpc_APS, forexample, when a value of the aps_params_type is 2, Qpc_data( ) may besignaled.

Semantics for syntax elements in Table 8 may be the same as thefollowing table.

TABLE 9 adaptation_parameter_set_id provides an identifier for the APSfor reference by other syntax elements.  NOTE - APSs can be sharedacross pictures and can be different in different slices  within apicture. aps_extension_flag equal to 0 specifies that noaps_extension_data_flag syntax elements are present in the APS RBSPsyntax structure. aps_extension_flag equal to 1 specifies that there areaps_extension_data_flag syntax elements present in the APS RBSP syntaxstructure. aps_extension_data_flag may have any value. Its presence andvalue do not affect decoder conformance to profiles specified in thisversion of this Specification. Decoders conforming to this version ofthis Specification shall ignore all aps_extension_data_flag syntaxelements. aps_params_type specifies the type of APS parameters carriedin the APS as specified in the Table 2 shown below.

For example, the syntax element adaptation parameter set id may providean identifier of an APS referred by other syntax elements.

Furthermore, for example, the syntax element aps_extension_flag mayrepresent whether aps_extension_data_flag syntax elements are present inan APS RBSP syntax structure. For example, the syntax elementaps_extension_flag having a value of 1 may represent that theaps_extension_data_flag syntax elements are present in the APS RBSPsyntax structure. The syntax element aps_extension_flag having a valueof 0 may represent that the aps_extension_data_flag syntax elements arenot present in the APS RBSP syntax structure.

Furthermore, for example, the syntax element aps_extension_data_flag mayhave any value. The presence (presence and value) of theaps_extension_data_flag may not affect decoding suitability for aprofile specified in a version of this standard. For example, a decodingapparatus that follows a version of this standard may ignore all syntaxelements aps_extension_data_flag.

Furthermore, for example, the syntax element aps_params_type mayrepresent the type of APS parameters included in an APS as illustratedin Table 10.

TABLE 10 Name of Type of APS aps_params_type aps_params_type parameters0 ALF_APS ALF parameters 1 LMCS_APS LMCS parameters 2 Qp_(C)_APS Qpcdata parameters 3 . . . 7 Reserved Reserved

For example, referring to Table 10, when a value of the syntax elementaps_params_type is 0, the syntax element aps_params_type may representthat the type of APS parameters is ALF parameters. When a value of thesyntax element aps_params_type is 1, the syntax element aps_params_typemay represent that the type of APS parameters is LMCS parameters. When avalue of the syntax element aps_params_type is 2, the syntax elementaps_params_type may represent that the type of APS parameters is Qpcdata parameters. The Qpc data parameter may represent a chromaquantization data parameter.

Furthermore, this document proposes another embodiment in whichinformation for a quantization parameter is signaled.

For example, the present embodiment proposes a scheme for signaling userdefined Qpc data in a picture parameter set (PPS). As an example forperforming a scheme proposed in the present embodiment, a flagrepresenting whether a PPS includes user defined data in an SPS may beintroduced. That is, a flag representing whether a PPS includes userdefined data in an SPS, may be signaled. Furthermore, according to thepresent embodiment, user defined data may be signaled in a PPS.Alternatively, user defined data may be signaled in a slice headerand/or another header set.

The flag representing whether a PPS includes user defined data may besignaled as in the following table.

TABLE 11 Descriptor seq_parameter_set_rbsp( ) { sps_decoding_parameter_set_id u(4) ... Qpc_data_default_flag u(1) ... rbsp_trailing_bits( ) }

For example, the syntax element Qpc_data_default_flag may be a syntaxelement of the aforementioned flag. The syntax elementQpc_data_default_flag may represent whether Qpc_data( ) parameters arepresent in a PPS RBSP syntax structure. For example,Qpc_data_default_flag of 0 may represent that Qpc_data( ) parameters arenot present in the PPS RBSP syntax structure and a default table is usedto help a determination of chroma quantization. In this case, thedefault table may be the same as Table 7. Furthermore, for example,Qpc_data_default_flag of 1 may represent that Qpc_data( ) parameters maybe present in the PPS RBSP syntax structure.

Furthermore, user defined data signaled in a PPS according to thepresent embodiment may be the same as the following table.

TABLE 12 Descriptor pic_parameter_set_rbsp( ) { pps_pic_parameter_set_id ue(v) ...  if(Qpc_data_default_flag)  Qpc_data( ) ...  rbsp_trailing_bits( ) }

Meanwhile, for example, Qpc_data( ) may include information necessaryfor chroma quantization derivation when ChromaArrayType is 1.

Furthermore, this document proposes another embodiment in whichinformation for a quantization parameter is signaled.

For example, the present embodiment proposes a flexible structure forchroma quantization parameter (QP) derivation and combined chroma QPderivation. The present embodiment proposes a scheme for signaling aninitial flag representing whether a user defined mode in whichparameters representing a function used to derive a chroma quantizationparameter (QP) in an SPS and/or a PPS may be used is present.

For example, flag information signaled in a high level syntax proposedin the present embodiment may be the same as a table to be describedlater.

TABLE 13 Descriptor high_level_syntax_parameter_set { ...Qpc_data_present_flag u(1) ... }

For example, Qpc_data_present_flag may represent whether parameters forderiving a chroma quantization parameter are present in a high levelsyntax RBSP syntax structure. For example, Qpc_data_present_flag havinga value of 0 may represent that chroma quantization parameters are notpresent in a high level syntax RBSP syntax structure. Furthermore, forexample, Qpc_data_present_flag having a value of 1 may represent thatchroma quantization parameters are present in a high level syntax RBSPsyntax structure.

Alternatively, the syntax element Qpc_data_present_flag may be used toindicate a scheme of using chroma quantization derivation in abitstream. For example, the syntax element Qpc_data_present_flag mayrepresent a tool used for chroma quantization derivation or the use of auser defined mode as follows.

For example, Qpc_data_present_flag may represent whether user definedchroma quantization is used in a bitstream. For example,Qpc_data_present_flag having a value of 0 may represent that userdefined chroma quantization is not used in a bitstream. Furthermore, forexample, Qpc_data_present_flag having a value of 1 may represent thatuser defined chroma quantization is used solely or along with anotherflag.

Furthermore, this document proposes another embodiment in whichinformation for a quantization parameter is signaled.

For example, the present embodiment proposes an embodiment in which howchroma quantization parameters (QP), that is, Qp′_(Cb), Qp′_(Cr) andQp′_(CbCr) can be derived using user defined information signaled in onefunction. For example, according to the present embodiment, datarepresenting a function for deriving a chroma quantization parameter(QP) may be signaled, and chroma quantization parameters may be derivedbased on chroma quantization data. Data (or user defined QP mappingtable) for chroma quantization parameter derivation may be signaled asin the following table.

TABLE 14 Descriptor Qp_(C)_data( ) {  qPi_min_idx ue(v) qPi_delta_max_idx ue(v)  for ( i = qPi_min_idx; i <= qPiMaxIdx; i++ )  Qp_(C)_qPi_val[ i ] ue(v)  QpOffset_(C) ue(v) }

Semantics for syntax elements in Table 14 may be the same as thefollowing table.

TABLE 15 qPi_min_idx specifies the minimum qPi index used in the chromaquantization. qPi_delta_max_idx specifies the delta value between theQp_(C —)min_idx and the maximum qPi index used for the chroma Qp_(C)derivation. The value of qPiMaxIdx shall be greater than or equal toqPi_min_idx. The maximum index qPiMaxIdx used in Qp_(C) derivation isderived as follows:  qPiMaxIdx = qPi_min_idx + qPi_delta_max_idxQp_(C —)qPi_val[i] specifies the Qp_(C) value for the i^(th) index.QpOffset_(C) specifies the offset value to be used the derivation of theQp_(C). The variable Qp_(C)Idx[ qPi ] for qPi with qPi = 0 ...qPiMaxIdx, is derived as follows: - For qPi < qPi_min_idx, Qp_(C)Idx[qPi] is set equal to qPi. - For qPi = qPi_min_idx ... qPiMaxIdx, thefollowing applies:   Qp_(C)Idx[ qPi ] = Qp_(C —)qPi_val[ qPi ] - ForqPi > qPiMaxIdx, Qp_(C)Idx[ qPi ] = qPi − QpOffset_(C) Tire value ofQp_(C) is derived as Qp_(C)Idx[qPi].

For example, the syntax element qPi_min_idx may represent a minimum qPiindex used in chroma quantization.

Furthermore, for example, the syntax element qPi_delta_max_idx mayrepresent a delta value between Qpi_min_idx and a maximum qPi index usedin chroma Qp_(c) derivation. A value of qPiMaxIdx may be greater than orequal to qPi_min_idx. For example, a maximum index qPiMaxIdx used inQp_(c) derivation may be derived as in the following equation.

qPiMaxIdx=qPi_min_idx+qPi_delta_max_idx  [Equation 4]

Furthermore, for example, the syntax element Qp_(C)_qPi_val[i] mayrepresent a Qp_(C) value for an i-th index.

Furthermore, for example, the syntax element QpOffset_(C) may representan offset value used for the derivation of Qp_(C).

Furthermore, for example, a parameter Qp_(C)Idx[qPi] for qPi may bederived as follows. In this case, the qPi may be 0 to qPiMaxIdx.

-   -   When qPi<qPi_min_idx, Qp_(C)Idx[qPi] may be set identically with        qPi.    -   When qPi=qPi_min_idx . . . qPiMaxIdx, Qp_(C)Idx[qPi] may be set        identically with Qp_(C)_qPi_val[qPi].    -   When qPi>qPiMaxIdx, Qp_(C)Idx[qPi] may be set as        qPi−QpOffset_(C).

Thereafter, a value of Qp_(C) may be derived as Qp_(C)Idx[qPi].

For example, according to the present embodiment, if a process ofderiving a quantization parameter is described in a standard format, theprocess may be represented as in the following table.

TABLE 16 Derivation process for quantization parameters Inputs to thisprocess are: -a luma location ( xCb, yCb ) specifying the top-left lumasample of the current coding block relative to the top-left luma sampleof the current picture, -a variable cbWidth specifying the width of thecurrent coding block in luma samples, -a variable cbHeight specifyingthe height of the current coding block in luma samples, -a variabletreeType specifying whether a single tree (SINGLE_TREE) or a dual treeis used to partition the CTUs and, when a dual tree is used, whether theluma (DUAL_TREE_LUMA) or chroma components (DUAL_TREE_CHROMA) arecurrently processed. In this process, the luma quantization parameterQp′_(Y) and the chroma quantization parameters Qp′_(Cb) and Qp′_(Cr) arederived. The luma location ( xQg, yQg ), specifies the top-left lumasample of the current quantization group relative to the top left lumasample of the current picture. The horizontal and vertical positions xQgand yQg are set equal to CuQgTopLeftX and CuQgTopLeftY, respectively.NOTE - : The current quantization group is a rectangluar region inside acoding tree block that shares the same qP_(Y) _(—) _(PRED). Its widthand height are equal to the width and height of the coding tree node ofwhich the top-left luma sample position is assigned to the variablesCuQgTopLeftX and CuQgTopLeftY. When treeType is equal to SINGLE_TREE orDUAL_TREE_LUMA, the predicted luma quantization parameter qP_(Y) _(—)_(PRED) is derived by the following ordered steps:  1. The variableqP_(Y) _(—) _(PREV) is derived as follows:   - If one or more of thefollowing conditions are true, qP_(Y) _(—) _(PREV) is set equal to  SliceQp_(Y):    -  The current quantization group is the firstquantization group in a slice.    -  The current quantization group isthe first quantization group in a brick.   - Otherwise, qP_(Y) _(—)_(PREV) is set 2. The availability derivation process for a block asspecified in clause 6.4.X [Ed. (BB): Neighbouring blocks availabilitychecking process tbd] is invoked with the location ( xCurr, yCurr ) setequal to ( xCb, yCb ) and the neighbouring location ( xNbY, yNbY ) setequal to ( xQg − 1, yQg ) as inputs, and the output is assigned toavailableA. The variable qP_(Y) _(—) _(A) is derived as follows: - Ifone or more of the following conditions are true, qP_(Y A) is set equalto qP_(Y PRFV): - availableA is equal to FALSE. - the CTB addressctbAddrA of the CTB containing the luma coding block covering the lumalocation ( xQg − 1, yQg ) is not equal to CtbAddrInBs, where ctbAddrA isderived as follows: xTmp = ( xQg − 1 ) >> MinTbLog2SizeY yTmp = yQg >>MinTbLog2SizeY minTbAddrA = MmTbAddrZs[ xTmp ][ yTmp ] ctbAddrA =minTbAddrA >> ( 2 * ( CtbLog2SizeY − MinTbLog2SizeY ) ) - Otherwise,qP_(Y) _(—) _(A) is set equal to the luma quantization parameter Qp_(Y)of the coding unit containing the luma coding block covering ( xQg − 1,yQg ). 3. The availability derivation process for a block as specifiedin clause 6.4.X [Ed. (BB): Neighbouring blocks availability checkingprocess tbd] is invoked with the location ( xCurr, yCurr ) set equal to( xCb, yCb ) and the neighbouring location ( xNbY, yNbY ) set equal to (xQg, yQg − 1 ) as inputs, and the output is assigned to availableB. Thevariable qP_(Y) _(—) _(B) is derived as follows: - If one or more of thefollowing conditions are true, qP_(Y B) is set equal to qP_(Y PREV): -availableB is equal to FALSE. - the CTB address ctbAddrB of the CTBcontaining the luma coding block covering the luma location ( xQg, yQg −1 ) is not equal to CtbAddrInBs, where ctbAddrB is derived as follows:xTmp = xQg >> MinTbLog2SizeY yTmp = ( yQg − 1 ) >> MinTbLog2SizeYminTbAddrB = MinTbAddrZs[ xTmp ][ yTmp ] ctbAddrB = minTbAddrB >> ( 2 *( CtbLog2SizeY − MinTbLog2SizeY ) ) (8-922) - Otherwise, qP_(Y) _(—)_(B) is set equal to the luma quantization parameter Qp_(Y) of thecoding unit containing the luma coding block covering ( xQg, yQg − 1 ).4. The predicted luma quantization parameter qP_(Y PRED) is derived asfollows: - If all the following conditions are true, then qP_(Y) _(—)_(PRED) is set equal to the luma quantization parameter Qp_(Y) of thecoding unit containing the luma coding block covering ( xQg, yQg − 1): - availableB is equal to TRUE. - the current quantization group isthe first quantization group in a CTB row within a brick - Otherwise,qP_(Y) _(—) _(PRED) is derived as follows: qP_(Y) _(—) _(PRED) = (qP_(Y) _(—) _(A) + qP_(Y) _(—) _(B) + 1 ) >> 1    The variable Qp_(Y) isderived as follows: Qp_(Y) = ( ( qP_(Y) _(—) _(PRED) + CuQpDeltaVal +64 + 2 * QpBdOffset_(Y) )%( 64 + QpBdOffset_(Y) ) ) − Qp BdOffset_(Y)   The luma quantization parameter Qp′_(Y) is derived as follows:Qp′_(Y) = Qp_(Y) + QpBdOffset_(Y)    When ChromaArrayType is not equalto 0 and treeType is equal to SINGLE_TREE or    DUAL_TREE_CHROMA, thefollowing applies:    - When treeType is equal to DUAL_TREE_CHROMA, thevariable Qp_(Y) is set equal to the luma quantization parameter Qp_(Y)of the luma coding unit that covers the luma location ( xCb + cbWidth /2, yCb + cbHeight / 2 ).    - The variables qP_(Cb), qP_(Cr) andqP_(CbCr) are derived as follows: qPi_(Cb) = Clip3( −QpBdOffset_(C), 69,Qp_(Y) + pps_cb_qp_offset + slice_cb_qp_offset ) qPi_(Cr) = Clip3(−QpBdOffset_(C), 69, Qp_(Y) + pps_cr_qp_offset + slice_cr_qp_offset )qPi_(CbCr) = Clip3( −QpBdOffset_(C), 69, Qp_(Y) +pps_joint_cbcr_qp_offset + slice_joint_c bcr_qp_offset ) -  IfChromaArrayType is equal to 1, the variables qP_(Cb), qP_(Cr) andqP_(CbCr) are set equal to the value of Qp_(C) as specified in-Table8-15 based on the index qPi equal to qPi_(Cb), qPi_(Cr) and qPi_(CbCr),respectively clause 7.x x    - Otherwise, the variables qP_(Cb), qP_(Cr)and qP_(CbCr) are set equal to Min( qPi, 63 ), based on the index qPiequal to qPi_(Cb), qPi_(Cr) and qPi_(CbCr), respectively.    - Thechroma quantization parameters for the Cb and Cr components, Qp′_(Cb)and Qp′_(Cr), and joint Cb-Cr coding Qp′_(CbCr) are derived as follows:Qp′_(Cb) = qP_(Cb) + QpBdOffset_(C) Qp′_(Cr) = qP_(Cr) + QpBdOffset_(C)Qp′_(CbCr) = qP_(CbCr) + QpBdOffset_(C)

Referring to Table 16, a derivation process for luma and chromaquantization parameter may be started based on the fact that an inputfor the process is a luma location (xCb, yCb), a parameter cbWidth andcbHeight designating the width and height of a current coding block, anda parameter treeType designating a single tree or a dual tree.Meanwhile, as in the aforementioned contents, a luma quantizationparameter and a chroma quantization parameter may be represented asQp′_(Y), Qp′_(Cb) and Qp′_(Cr).

Furthermore, this document proposes another embodiment in whichinformation for a quantization parameter is signaled.

For example, the present embodiment proposes an example in which a flagwithin as an SPS has a user defined mode or a default mode, syntaxelements which may be used to control the derivation of a quantizationparameter are used. An example of a syntax element which may be used toderive a quantization parameter may be the same as the following tables.Meanwhile, a structure of the syntax elements is not limited to astructure illustrated in the following tables, for example.

TABLE 17 Descriptor pic_parameter_set_rbsp( ) { pps_pic_parameter_set_id ue(v) ...  Qpc_data_default_flag u(1) ...

TABLE 18 Descriptor slice_header( ) {  slice_pic_parameter_set_id ue(v)...  if(!Qpc_data_default_flag )   slice_Qp_(C)_aps_id u(5) byte_alignment( ) }

TABLE 19 Descriptor adaptation_parameter_set_rbsp( ) { adaptation_parameter_set_id u(5)  aps_params_type u(3)  if(aps_params_type = = ALF_APS )   alf_data(adaptation_parameter_set_id ) else if ( aps_params_type = = LMCS_APS )   lmcs_data( )  else if (aps_params_type = = Qpc_APS ) // 2   Qp_(C)_data( )  aps_extension_flagu(1)  if( aps_extension_flag )   while( more_rbsp_data( ) )   aps_extension_data_flag u(1)  rbsp_trailing_bits( ) }

For example, the syntax element Qpc_data_default_flag may representwhether a user defined mode is used for the derivation of a quantizationparameter. For example, Qpc_data_default_flag having a value of 0 mayrepresent that the user defined mode is used for the derivation of aquantization parameter. Furthermore, for example, Qpc_data_default_flaghaving a value of 1 may represent that a default table is used for thederivation of a chroma quantization parameter. In this case, the defaulttable may be the same as Table 7. Furthermore, if the syntax elementQpc_data_default_flag is not present, the syntax elementQpc_data_default_flag may be inferred as 1.

Meanwhile, if the user defined mode is used, a corresponding sliceheader, tile group/header, or another proper header may be used tosignal an APS ID. For example, as in Table 18, a syntax elementrepresenting an APS ID through a slice header may be signaled.

For example, the syntax element slice_Qp_(c)_aps_id may representadaptation_parameter_set_id of a Qp_(c) APS referred to by a slice.TemporalId of a Qp_(c) APS NAL unit having adaptation_parameter_set_id,such as slice_Qp_(c)_aps_id, may be smaller than or equal to TemporalIdof a coded slice NAL unit. If a plurality of Qp_(c) APSs havingadaptation_parameter_set_id having the same value is referred to by twoor more slices of the same picture, a plurality of Qp_(c) APSs havingadaptation_parameter_set_id having the same value may have the samecontent.

Furthermore, an APS structure that transfers chroma quantization data,which is proposed in the present embodiment, may be the same as Table19.

For example, the syntax element adaptation_parameter_set_id may providean identifier of an APS referred to by other syntax elements.

Furthermore, for example, the syntax element aps_extension_flag mayrepresent whether aps_extension_data_flag syntax elements are present inan APS RBSP syntax structure. For example, the syntax elementaps_extension_flag having a value of 1 may represent that theaps_extension_data_flag syntax elements are present in an APS RBSPsyntax structure. The syntax element aps_extension_flag having a valueof 0 may represent that the aps_extension_data_flag syntax elements arenot present in an APS RBSP syntax structure.

Furthermore, for example, the syntax element aps_extension_data_flag mayhave any value. The presence (presence and value) of theaps_extension_data_flag may not affect decoding suitability for aprofile specified in a version of this standard. For example, thedecoding apparatus following a version of this standard may ignore allof the syntax elements aps_extension_data_flag.

Furthermore, for example, the syntax element aps_params_type mayrepresent the type of APS parameters included in an APS as illustratedin Table 10.

Qp_(c)_data( ) disclosed in Table 19 may be signaled as in the followingtable.

TABLE 20 Descriptor Qp_(C)_data ( ) {   qPi_min_idx ue(v)  qPi_delta_max_idx ue(v)  Qp_(C)_prec_minus1 ue(v)  Qp_(C)_init_value(v)   for ( i = qPi_min_idx + 1; i <= qPiMaxIdx; i++ )  Qp_(C)_qPi_delta_val[ i ] ue(v)   QpOffset_(C) ue(v) }

For example, the syntax element qPi_min_idx may represent a minimum qPiindex used in chroma quantization.

Furthermore, for example, the syntax element qPi_delta_max_idx mayrepresent a delta value between Qpi_min_idx and a maximum qPi index usedin chroma Qp_(c) derivation. A value of qPiMaxIdx may be greater than orequal to qPi_min_idx. For example, a maximum index qPiMaxIdx used inQp_(c) derivation may be derived like Equation 4.

Furthermore, for example, a value obtained by adding 1 to the syntaxelement Qp_(c)_prec_minus1 may represent the number of bits used for therepresentation of the syntax lmcs_delta_abs_cw[i]. A value ofQp_(c)_prec_minus1 may be in a range of 0 to BitDepth_(Y)−2.

Furthermore, for example, the syntax element Qp_(c)_init_val mayrepresent a Qp_(C) value corresponding to qPi_min_idx.

Furthermore, for example, the syntax element Qp_(C)_qPi_delta_val[i] mayrepresent the delta of a Qp_(C) value for an i-th index.

Furthermore, for example, the syntax element QpOffset_(C) may representan offset value used for the derivation of Qp_(c).

For example, a parameter Qp_(C)Idx[qPi] for qPi may be derived asfollows. In this case, the qPi may be 0 to qPiMaxIdx.

-   -   When qPi<qPi_min_idx, Qp_(C)Idx[qPi] may be set identically with        qPi.    -   When qPi=qPi_min_idx . . . qPiMaxIdx, Qp_(C)Idx[qPi] may be set        as Qp_(c)_qPi_delta_val[qPi]+Qp_(C)Idx[qPi−1].    -   When qPi>qPiMaxIdx, Qp_(C)Idx[qPi] may be set as        qPi−QpOffset_(C).

Thereafter, a value of Qp_(C) may be derived as Qp_(C)Idx[qPi].

As in the aforementioned embodiment, a chroma quantization parameter,that is, Qp′Cb, Qp′Cr, and Qp′CbCr, may be derived using signaled userdefined information or a default value illustrated in a default table,such as Table 7.

For example, in the present embodiment, if a process of deriving aquantization parameter is written in a standard format, the process maybe represented as in the following table.

TABLE 21 Derivation process for quantization parameters Inputs to thisprocess are: -a luma location ( xCb, yCb ) specifying the top-left lumasample of the current coding block relative to the top-left luma sampleof the current picture, -a variable cbWidth specifying the width of thecurrent coding block in luma samples, -a variable cbHeight specifyingthe height of the current coding block in luma samples, -a variabletreeType specifying whether a single tree (SINGLE_TREE) or a dual treeis used to partition the CTUs and, when a dual tree is used, whether theluma (DUAL_TREE_LUMA) or chroma components (DUAL_TREE_CHROMA) arecurrently processed. In this process, the luma quantization parameterQp′_(Y) and the chroma quantization parameters Qp′_(Cb) and Qp′_(Cr) arederived. The luma location ( xQg, yQg ), specifies the top-left lumasample of the current quantization group relative to the top left lumasample of the current picture. The horizontal and vertical positions xQgand yQg are set equal to CuQgTopLeftX and CuQgTopLeftY, respectively.NOTE - : The current quantization group is a rectangluar region inside acoding tree block that shares the same qP_(Y) _(—) _(PRED). Its widthand height are equal to the width and height of the coding tree node ofwhich the top-left luma sample position is assigned to the variablesCuQgTopLeftX and CuQgTopLeftY. When treeType is equal to SINGLE_TREE orDUAL_TREE_LUMA, the predicted luma quantization parameter qP_(Y) _(—)_(PRED) is derived by the following ordered steps: 1. The variableqP_(Y) _(—) _(PREV) is derived as follows: -  If one or more of thefollowing conditions are true, qP_(Y) _(—) _(PREV) is set equal toSliceQp_(Y): - The current quantization group is the first quantizationgroup in a slice. - The current quantization group is the firstquantization group in a brick. -  Otherwise, qP_(Y) _(—) _(PREV) is set2. The availability derivation process for a block as specified inclause 6.4.X [Ed. (BB): Neighbouring blocks availability checkingprocess tbd] is invoked with the location ( xCurr, yCurr ) set equal to( xCb, yCb ) and the neighbouring location ( xNbY, yNbY ) set equal to (xQg − 1, yQg ) as inputs, and the output is assigned to availableA. Thevariable qP_(Y) _(—) _(A) is derived as follows: -  If one or more ofthe following conditions are true, qP_(Y) _(—) _(A) is set equal toqP_(Y) _(—) _(PREV):  - availableA is equal to FALSE.  - the CTB addressctbAddrA of the CTB containing the luma coding block covering the lumalocation ( xQg − 1, yQg ) is not equal to CtbAddrInBs, where ctbAddrA isderived as follows: xTmp = ( xQg − 1 ) >> MinTbLog2SizeY yTmp = yQg >>MinTbLog2SizeY minTbAddrA = MinTbAddrZs[ xTmp ][ yTmp ] ctbAddrA =minTbAddrA >> ( 2 * ( CtbLog2SizeY − MinTbLog2SizeY ) ) -  Otherwise,qP_(Y) _(—) _(A) is set equal to the luma quantization parameter Qp_(Y)of the  coding unit containing the luma coding block covering ( xQg − 1,yQg ). 3. The availability derivation process for a block as specifiedin clause 6.4.X [Ed. (BB): Neighbouring blocks availability checkingprocess tbd] is invoked with the location ( xCurr, yCurr ) set equal to( xCb, yCb ) and the neighbouring location ( xNbY, yNbY ) set equal to (xQg, yQg − 1 ) as inputs, and the output is assigned to availableB. Thevariable qP_(Y) _(—) _(B) is derived as follows: -  If one or more ofthe following conditions are true, qP_(Y B) is set equal to qP_(Y PREV): - availableB is equal to FALSE.  - the CTB address ctbAddrB of the CTBcontaining the luma coding block covering the luma location ( xQg, yQg −1 ) is not equal to CtbAddrInBs, where ctbAddrB is derived as follows:xTmp = xQg >> MinTbLog2SizeY yTmp = ( yQg − 1 ) >> MinTbLog2SizeYminTbAddrB = MinTbAddrZs[ xTmp ][ yTmp ] ctbAddrB = minTbAddrB >> ( 2 *( CtbLog2SizeY − MinTbLog2SizeY ) ) (8-922) -  Otherwise, qP_(Y B) isset equal to the luma quantization parameter Qp_(Y) of the  coding unitcontaining the luma coding block covering ( xQg, yQg − 1 ). 4. Thepredicted luma quantization parameter qP_(Y) _(—) _(PRED) is derived asfollows: -  If all the following conditions are true, then qP_(Y PRED)is set equal to the luma  quantization parameter Qp_(Y) of the codingunit containing the luma coding block  covering ( xQg, yQg − 1):  -availableB is equal to TRUE.  - the current quantization group is thefirst quantization group in a CTB row within a brick -  Otherwise,qP_(Y PRFD) is derived as follows: qPy_pred = ( qP_(Y) _(—) _(A) +qP_(Y) _(—) _(B) + 1 ) >> 1 The variable Qp_(Y) is derived as follows:QP_(Y) = ( ( qP_(Y) _(—) _(PRED) + CuQpDeltaVal + 64 + 2 *QpBdOffset_(Y) )%( 64 + QpBdOffset_(Y) ) ) − Qp BdOffset_(Y) The lumaquantization parameter Qp′_(Y) is derived as follows: Qp′_(Y) =Qp′_(Y) + QpBdOffset_(Y) When ChromaArrayType is not equal to 0 andtreeType is equal to SINGLE_TREE or DUAL_TREE_CHROMA, the followingapplies: - When treeType is equal to DUAL_TREE_CHROMA, the variableQp_(Y) is set equal to the luma quantization parameter Qp_(Y) of theluma coding unit that covers the luma location ( xCb + cbWidth / 2,yCb + cbHeight / 2 ). - The variables qP_(Cb), qP_(Cr) and qP_(CbCr) arederived as follows: qPi_(Cb) = Clip3( −QpBdOffset_(C), 69, Qp_(Y) +pps_cb_qp_offset + slice_cb_qp_offset ) qPi_(Cr) = Clip3(−QpBdOffsct_(C), 69, Qp_(Y) + pps_cr_qp_offset + slice_cr_qp_offset )qPi_(CbCr) = Clip3( −QpBdOffset_(C), 69, Qp_(Y) +pps_joint_cbcr_qp_offset + slice_joint_c bcr_qp_offset ) - IfChromaArrayType is equal to 1, and Qpc_data_default_flag is equal toFALSE, the variables qP_(Cb), qP_(Cr) and qP_(CbCr) are set equal to thevalue of Qp_(C) as specified in clause 7.x.x - If ChromaArrayType isequal to 1, and Qpc_data_default_flag is equal to TRUE, the variablesqP_(Cb), qP_(Cr) and qP_(cbCr) are specified in Table 8-15 based on theindex qPi equal to qPi_(Cb), qPi_(Cr) qPi_(CbCr), respectively -Otherwise, the variables qP_(Cb), qP_(Cr), and qP_(CbCr) are set equalto Min( qPi, 63 ), based on the index qPi equal to qPi_(Cb), qPi_(Cr)and qPi_(CbCr), respectively. - The chroma quantization parameters forthe Cb and Cr components, Qp′_(Cb) and Qp′_(Cr), and joint Cb-Cr codingQp′_(CbCr) are derived as follows: Qp′_(Cb) = qP_(Cb) + QpBdOffset_(C)Qp′_(Cr) = qP_(Cr) + QpBdOffset_(C) Qp′_(CbCr) = qP_(CbCr) +QpBdOffset_(C) Table 8-15 - Specification of Qp_(C) as a function of qPifor ChromaArrayType equal to 1 qPi < 30 30 31 32 33 34 35 36 37 38 39 4041 42 43 > 43 Qp_(C) = qPi 29 30 31 32 33 33 34 34 35 35 36 36 37 37=qPi − 6

Referring to Table 21, when ChromaArrayType is 1 andQp_(c)_data_default_flag indicates false (i.e., for example, whenQp_(c)_data_default_flag is 0), the parameters qP_(Cb), qP_(Cr) andqP_(CbCr) may be derived based on signaled user defined information asproposed in the present embodiment. When ChromaArrayType is 1 andQp_(c)_data_default_flag indicates true (i.e., for example, whenQp_(c)_data_default_flag is 1), the parameters qP_(Cb), qP_(Cr) andqP_(CbCr) may be derived by a default table based on the same indicesqPi_(Cb), qPi_(Cr) and qPi_(CbCr), respectively.

Furthermore, this document proposes another embodiment in whichinformation for a quantization parameter is signaled.

For example, the present embodiment proposes syntax elements which maybe used to control the derivation of a quantization parameter byindicating a user defined mode or a default mode through the flag of anSPS. Specifically, the present embodiment proposes a scheme forsignaling the syntax elements of the following syntax structure.Meanwhile, a structure of the syntax elements is an example, and is notlimited to a structure illustrated in the following table.

TABLE 22 Descriptor Qp_(C)_data ( ) {  qPi_min_idx ue(v) qPi_delta_max_idx ue(v)  for ( i = qPi_min_idx; i <= qPiMaxIdx, i++ )  Qp_(C)_qPi_delta_val[ i ] ue(v)   QpOffset_(C) ue(v) }

For example, the syntax element qPi_min_idx may represent a minimum qPiindex used in chroma quantization.

Furthermore, for example, the syntax element qPi_delta_max_idx mayrepresent a delta value between Qpi_min_idx and a maximum qPi index usedin chroma Qp_(c) derivation. A value of qPiMaxIdx may be greater than orequal to qPi_min_idx. For example, maximum index qPiMaxIdx used inQp_(c) derivation may be derived like Equation 4.

Furthermore, for example, the syntax element Qp_(C)_qPi_delta_val[i] mayrepresent the delta of a Qp_(C) value for an i-th index.

Furthermore, for example, the syntax element QpOffset_(C) may representan offset value used in the derivation of Qp_(c), such as theaforementioned contents.

As in the aforementioned embodiment, chroma quantization parameter, thatis, Qp′Cb, Qp′Cr and Qp′CbCr may be derived using signaled user definedinformation or a default value used in a default table, such as Table 7.

For example, in the present embodiment, if a process of deriving aquantization parameter is written in a standard format, the process maybe represented as in the following table.

TABLE 23 Derivation process for quantization parameters Inputs to thisprocess are: -a luma location ( xCb, yCb ) specifying the top-left lumasample of the current coding block relative to the top-left luma sampleof the current picture. -a variable cbWidth specifying the width of thecurrent coding block in luma samples, -a variable cbHeight specifyingthe height of the current coding block in luma samples, -a vanabletreeType specifying whether a single tree (SINGLE_TREE) or a dual treeis used to partition the CTUs and, when a dual tree is used, whether theluma (DUAL_TREE_LUMA) or chroma components (DUAL_TREE_CHROMA) arecurrently processed. hi this process, the luma quantization parameterQp′_(Y) and the chroma quantization parameters Qp′_(Cb), and Qp′_(Cr)are derived The luma location (xQg, vQg ). specifics the top-left lumasample of the current quantization group relative to the top left lumasample of the current picture. The horizontal and vertical positions xQgand yQg are set equal to CuQgTopLeftX and CuQgTopLeftY, respectively.NOTE - : The current quantization group is a rectangluar region inside acoding tree block that shares the same qP_(Y) _(—) _(PRED). Its widthand height are equal to the width and height of the coding tree node ofwhich the top-left luma sample position is assigned to the variablesCuQgTopLeftX and CuQgTopLeftY. When treeType is equal to SINGLE_TREE orDUAL_TREE_LUMA, the predicted luma quantization parameter qP_(Y) _(—)_(PRED) is derived by the following ordered steps: 1. The variableqP_(Y) _(—) _(PREV) is derived as follows: -  If one or more of thefollowing conditions are true, qP_(Y PREV) is set equal toSliceQp_(Y): - The current quantization group is the first quantizationgroup in a slice. - The current quantization group is the firstquantization group in a brick. -  Otherwise, qP_(Y) _(—) _(PREV) is set2. The availability derivation process for a block as specified inclause 6.4.X [Ed. (BB): Neighbouring blocks availability checkingprocess tbdj is invoked with the location ( xCurr, yCurr ) set equal to( xCb, yCb ) and the neighbouring location ( xNbY, yNbY ) set equal to (xQg − 1, yQg ) as inputs, and the output is assigned to availableA. Thevariable qP_(Y) _(—) _(A) is derived as follows: -  If one or more ofthe following conditions are true, qP_(Y A) is set equal to qP_(Y PRFV): - availableA is equal to FALSE.  - the CTB address ctbAddrA of the CTBcontaining the luma coding block covering the luma location ( xQg − 1,yQg ) is not equal to CtbAddrInBs, where ctbAddrA is derived as follows:xTmp = ( xQg − 1 ) >> MinTbLogzSizeY yTmp = yQg >> MinTbLog2SizeYminTbAddrA = MmTbAddrZs[ xTmp ][ yTmp ] ctbAddrA = minTbAddrA >> (2 * (CtbLog2SizeY − MinTbLog2SizeY ) ) -  Otherwise, qP_(Y) _(—) _(A) is setequal to the luma quantization parameter Qp_(Y) of the  coding unitcontaining the luma coding block covering ( xQg − 1, yQg ). 3. Theavailability derivation process for a block as specified in clause 6.4.X[Ed. (BB): Neighbouring blocks availability checking process tbd] isinvoked with the location ( xCurr, yCurr ) set equal to ( xCb, yCb ) andthe neighbouring location ( xNbY, yNbY ) set equal to ( xQg, yQg − 1 )as inputs, and the output is assigned to availableB. The variable qP_(Y)_(—) _(B) is derived as follows: -  If one or more of the follow ingconditions are true, qP_(Y B) is set equal to qP_(Y PREV):  - availableBis equal to FALSE.  - the CTB address ctbAddrB of the CTB containing theluma coding block covering the luma location (xQg, yQg − 1 ) is notequal to CtbAddrInBs, where ctbAddrB is derived as follows: xTmp =xQg >> MinTbLog2SizeY yTmp = ( vQg − 1 ) >> MinTbLog2SizeY minTbAddrB =MinTbAddrZs[ xTmp ][ yTmp ] ctbAddrB = minTbAddrB >> ( 2 * (CtbLog2SizeY − MinTbLog2SizeY ) ) (8-922) -  Otherwise, qP_(Y B) is setequal to the luma quantization parameter Qp_(Y) of the coding unitcontaining the luma coding block covering ( xQg, yQg − 1 ). 4. Thepredicted luma quantization parameter qP_(Y) _(—) _(PRED) is derived asfollows: -  If all the following conditions are true, then qP_(Y) _(—)_(PRED) is set equal to the luma  quantization parameter Qp_(Y) of thecoding unit containing the luma coding block  covering ( xQg, yQg − 1): - availableB is equal to TRUE.  - the current quantization group is thefirst quantization group in a CTB row within a brick -  Otherwise,qP_(Y) _(—) _(PRED) is derived as follows: qP_(Y PRED) ⁼ (qP_(Y A) +qP_(Y B) + 1 ) >> 1 The variable Qp_(Y) is derived as follows: Qp_(Y) =( ( qP_(Y) _(—) _(PRED) + CuQpDeltaVal + 64 + 2 * QpBdOffset_(Y) )%(64 + QpBdOffset_(Y) ) ) − Qp BdOffset_(Y) The luma quantizationparameter Qp′_(Y) is derived as follows: Qp′_(Y) = QP_(Y) +QpBdOffset_(Y) When ChromaArrayType is not equal to 0 and tree Type isequal to SINGLE_TREE or DUAL_TREE_CHROMA, the following applies: - WhentreeType is equal to DUAL_TREE_CHROMA, the variable Qp_(Y) is set equalto the luma quantization parameter Qp_(Y) of the luma coding unit thatcovers the luma location (xCb + cbWidth / 2, yCb + cbHeight / 2 ). - Thevariables qP_(Cb), qP_(Cr) and qP_(C)bCr ^(are) derived as follows:qPi_(Cb) = Clip3( −QpBdOffset_(C), 69, Qp_(Y) + pps_cb_qp_offset +slice_cb_qp_offset ) qPi_(Cr) = Clip3( −QpBdOffset_(C), 69, Qp_(Y) +pps_cr_qp_offset + slice_cr_qp_offset ) qPi_(CbCr) = Clip3(−QpBdOffset_(C), 69, Qp_(Y) + pps_joint_cbcr_qp_offset + slice_joint_cbcr_qp_offset ) - If ChromaArrayType is equal to 1, andQpc_data_default_flag is equal to FALSE, the variables qP_(Cb), qP_(Cr)and qP_(CbCr) are set equal to the value of Qp_(C) based on the indexqPi equal to qPi_(Cb), qPi_(Cr) and qPi_(CbCr), respectively asdescribed below: The variable Qp_(C)Idx[ i ] is derived as follows: -For i < qPi_min_idx, Qp_(C)Idx[ i ] is set equal to qPi. - For i =qPi_min_idx ... qPiMaxIdx, the following applies: Qp_(C)Idx[ i ] =Qp_(C —)qPi_delta_val[ i ] + Qp_(C)Idx[ i −1]  For i > qPiMaxIdx,Qp_(C)Idx[ i ] = qPi − QpOffset_(C) The value of Qp_(C) is derived asQp_(C)Idx[qPi] - If ChromaArrayType is equal to 1, andQpc_data_default_flag is equal to TRUE, the variables qP_(Cb), qP_(Cr)and qP_(CbCr) are specified in Table 8-15 based on the index qPi equalto qPi_(Cb), qPi_(Cr)and qPi_(CbCr), respectively - Otherwise, thevariables qP_(Cb,) qP_(Cr) and qP_(Cbcr) arc set equal to Min( qPi, 63), based on the index qPi equal to qPi_(Cb), qPi_(Cr) and qPi_(CbCr),respectively. - The chroma quantization parameters for the Cb and Crcomponents, Qp′_(Cb) and Qp′_(Cr), and joint Cb-Cr coding Qp′_(CbCr) arederived as follows: Qp′_(Cb) = qP^(Cb) + QpBdOffset_(C) Qp′_(Cr) =qP_(Cr) + QpBdOffset_(C) Qp′_(CbCr) = qP^(CbCr) + QpBdOffset_(C) Table8-15 - Specification of Qp_(C) as a function of qPi for ChromaArrayTypeequal to 1 qPi <30 30 31 32 33 34 35 36 37 38 39 40 41 42 43 > 43 Qp_(C)= qPi 29 30 31 32 33 33 34 34 35 35 36 36 37 37 = qPi − 6

Referring to Table 23, when ChromaArrayType is 1 andQp_(c)_data_default_flag indicates false (i.e., for example, whenQp_(c)_data_default_flag is 0), the parameters qP_(Cb), qP_(Cr) andqP_(CbCr) may be derived based on signaled user defined information asproposed in the present embodiment. For example, when ChromaArrayType is1 and Qp_(c)_data_default_flag indicates false (i.e., for example, whenQp_(c)_data_default_flag is 0), the parameters qP_(Cb), qP_(Cr) andqP_(CbCr) may be derived identically with a value of Qp_(C) based on thesame indices qPi as qPi_(Cb), qPi_(Cr) and qPi_(CbCr), respectively, asfollows.

For example, the parameter Qp_(C)Idx[i] may be derived as follows.

-   -   When i<qPi_min_idx, Qp_(C)Idx[qPi] may be set identically with        qPi.    -   When i=qPi_min_idx . . . qPiMaxIdx, Qp_(C)Idx[i] may be set as        Qp_(C)_qPi_delta_val[i]+Qp_(C)Idx[i−1].    -   When i>qPiMaxIdx, Qp_(C)Idx[i] may be set as qPi−QpOffset_(C).

Thereafter, the Qp_(C) may be set as the Qp_(C)Idx[i].

Furthermore, referring to Table 23, when ChromaArrayType is 1 andQp_(c)_data_default_flag indicates true (i.e., for example, whenQp_(c)_data_default_flag is 1), the parameters qP_(Cb), qP_(Cr) andqP_(CbCr) may be derived by a default table based on the same indicesqPi as qPi_(Cb), qPi_(Cr) and qPi_(CbCr), respectively.

Furthermore, this document proposes another embodiment in whichinformation for a quantization parameter is signaled.

For example, the present embodiment proposes syntax elements for achroma quantization (Qp_(C)) derivation parameter in an adaptationparameter set (APS). For example, an APS ID may be signaled in a sliceheader. Furthermore, for example, there may be proposed a flag within apicture parameter set (PPS) that represents whether a default table isused or a table derived from information signaled in an APS is used.Furthermore, for example, if a default table is not used, an additionalcontrol scheme for supporting access to an APS including Qp_(C) data maybe added to a slice header.

Meanwhile, according to the existing video/image standard, chroma QP maybe derived from luma QP, and may be updated by an additionally signaledchroma QP offset. The existing chroma quantization parameter Qp_(C)table may be a default table, such as Table 7.

The present embodiment proposes adding a function for signaling a chromaquantization parameter Qp_(C) as a function of an index qPi. An APS maybe used to integrate signaling schemes of Qp_(C) values.

For example, an APS according to the present embodiment may be the sameas the following table.

TABLE 24 Descriptor adaptation_parameter_set_rbsp( ) { adaptation_parameter_set_id u(5)  aps_params_type u(3)  if(aps_params_type = = ALF_APS )   alf_data(adaptation_parameter_set_id ) else if ( aps_params_type = = LMCS_APS )   lmcs_data( )  else if (aps_params_type = = Qp_(C)_APS ) // 2   Qp_(C)_data( ) aps_extension_flag u(1)  if( aps_extension_flag )   while(more_rbsp_data( ) )    aps_extension_data_flag u(1)  rbsp_trailing_bits() }

For example, the syntax element adaptation_parameter_set_id may providean identifier of an APS referred to by other syntax elements.

Furthermore, for example, the syntax element aps_params_type mayrepresent the type of APS parameters included in an APS as illustratedin Table 10.

Furthermore, for example, the syntax element aps_extension_flag mayrepresent whether the aps_extension_data_flag syntax elements arepresent in an APS RBSP syntax structure. For example, the syntax elementaps_extension_flag having a value of 1 may represent that theaps_extension_data_flag syntax elements are present in an APS RBSPsyntax structure. The syntax element aps_extension_flag having a valueof 0 may represent that the aps_extension_data_flag syntax elements arenot present in an APS RBSP syntax structure.

Furthermore, for example, the syntax element aps_extension_data_flag mayhave any value. The presence (presence and value) of theaps_extension_data_flag may not affect decoding suitability for aprofile specified in a version of this standard. For example, thedecoding apparatus following a version of this standard may ignore allof the syntax elements aps_extension_data_flag.

Qp_(c)_data( ) disclosed in Table 24 may be signaled as in the followingtable.

TABLE 25 Descriptor Qp_(C)_data ( ) {  qPi_min_idx ue(v) qPi_delta_max_idx ue(v)  for ( i = qPi_min_idx; i <= qPiMaxIdx; i++ )  Qp_(C)_qPi_delta_val[ i ] ue(v)  Qp_(C)Offset_(C)_present_flag u(1) if(Qp_(C)Offset_(C)_present_flag)   QpOffset_(C) ue(v) }

For example, the syntax element qPi_min_idx may represent a minimum qPiindex used in chroma quantization. A value of qPi_min_idx may be in arange of 0 to 63.

Furthermore, for example, the syntax element qPi_delta_max_idx mayrepresent a delta value between Qpi_min_idx and a maximum qPi index usedin chroma Qp_(c) derivation. A value of qPiMaxIdx may be greater than orequal to qPi_min_idx. Furthermore, for example, a value ofqPi_delta_max_idx may be in a range of 0 to 63. For example, a maximumindex qPiMaxIdx used in Qp_(c) derivation may be derived like Equation4.

Furthermore, for example, the syntax element Qp_(C)_qPi_delta_val[i] mayrepresent a difference between Qp_(C) values for an i-th index. The adifference may also be called a delta.

Furthermore, for example, the syntax elementQp_(C)Offset_(C)_present_flag may represent whether QpOffset_(C) ispresent in a bitstream. For example, Qp_(C)Offset_(C)_present_flaghaving a value of 1 may represent that QpOffset_(C) is present in abitstream. Furthermore, for example, Qp_(C)Offset_(C)_present_flaghaving a value of 0 may represent that QpOffset_(C) is not present in abitstream. When Qp_(C)Offset_(C)_present_flag is not present,Qp_(C)Offset_(C)_present_flag may be inferred as 0.

Furthermore, for example, the syntax element QpOffset_(C) may representan offset value used in the derivation of Qp_(C).

For example, a parameter Qp_(C)Idx[qPi] for qPi may be derived asfollows. In this case, the qPi may be 0 to 63.

-   -   When qPi<qPi_min_idx, Qp_(C)Idx[qPi] may be set identically with        qPi.    -   When qPi=qPi_min_idx . . . qPiMaxIdx, Qp_(C)Idx[qPi] may be set        as Qpc_qPi_delta_val[qPi]+Qp_(C)Idx[qPi−1].    -   If qPi>qPiMaxIdx, when Qp_(C)Offset_(C)_present_flag is 1,        Qp_(C)Idx[qPi] may be set as qPi−QpOffset_(C). If        Qp_(C)Offset_(C)_present_flag is not 1, that is, if        Qp_(C)Offset_(C)_present_flag is 0, Qp_(C)Idx[qPi] may be set as        qPi−(qPiMaxIdx−Qp_(C)Idx[qPiMaxIdx]).

Thereafter, a value of Qp_(C) may be derived as Qp_(C)Idx[qPi].

Furthermore, the present embodiment proposes a flag signaled as a PPS asin the following table.

TABLE 26 Descriptor pic_parameter_set_rbsp( ) { pps_pic_parameter_set_id ue(v) ...  Qpc_data_default_flag u(1) ...

For example, the syntax element Qp_(c)_data_default_flag may representwhether a user defined mode is used for quantization parameterderivation. For example, Qp_(c)_data_default_flag having a value of 0may represent that a user defined mode is used for quantizationparameter derivation. Furthermore, for example, Qp_(c)_data_default_flaghaving a value of 1 may represent that the aforementioned default tableis used for quantization parameter derivation. The default table may bethe same as Table 7. If Qp_(c)_data_default_flag is not present,Qp_(c)_data_default_flag may be inferred as 1.

Furthermore, the present embodiment proposes a syntax element signaledas a slice header as in the following table.

TABLE 27 Descriptor slice_header( ) {  slice_pic_parameter_set_id ue(v)...   if(!Qpc_data_default_flag )   slice_Qp_(C)_aps_id u(5) ... byte_alignment( ) }

For example, the syntax element slice_Qp_(c)_aps_id may representadaptation_parameter_set_id of a Qp_(c) APS referred to by a slice.TemporalId of a Qp_(c) APS NAL unit having adaptation_parameter_set_id,such as slice_Qp_(c)_aps_id, may be smaller than or the same asTemporalId of a coded slice NAL unit. If a plurality of Qp_(c) APSshaving adaptation_parameter_set_id having the same value is referred toby two or more slices of the same picture, a plurality of Qp_(c) APSshaving adaptation_parameter_set_id having the same value may have thesame content.

For example, in the present embodiment, if a process of deriving aquantization parameter is written in a standard format, the process maybe represented as in the following table.

TABLE 28 8.7.1 Derivation process for quantization parameters Inputs tothis process are: -a luma location ( xCb, yCb ) specifying the top-leftluma sample of the current coding block relative to the top-left lumasample of the current picture, -a variable cbWidth specifying the widthof the current coding block in luma samples, -a variable cbHeightspecifying the height of the current coding block in luma samples, -avariable treeType specifying whether a single tree (SINGLE_TREE) or adual tree is used to partition the CTUs and, when a dual tree is used,whether the luma (DUAL_TREE_LUMA) or chroma components(DUAL_TREE_CHROMA) are currently processed. In this process, the lumaquantization parameter Qp′_(Y) and the chroma quantization parametersQp′_(Cb) and Qp′_(Cr) are derived. The luma location ( xQg, yQg ),specifies the top-left luma sample of the current quantization grouprelative to the top left luma sample of the current picture. Thehorizontal and vertical positions xQg and yQg are set equal toCuQgTopLeftX and CuQgTopLeftY, respectively. NOTE - : The currentquantization group is a rectangluar region inside a coding tree blockthat shares the same qP_(Y) _(—) _(PRED). Its width and height are equalto the width and height of the coding tree node of which the top-leftluma sample position is assigned to the variables CuQgTopLeftX andCuQgTopLeftY. When treeType is equal to SINGLE_TREE or DUAL_TREE_LUMA,the predicted luma quantization parameter qP_(Y) _(—) _(PRED) is derivedby the following ordered steps: 1.The variable qP_(Y) _(—) _(PREV) isderived as follows: - If one or more of the following conditions aretrue, qP_(Y) _(—) _(PREV) is set equal to SliceQp_(Y): - The currentquantization group is the first quantization group in a slice. - Thecurrent quantization group is the first quantization group in a brick. -Otherwise, qP_(Y) _(—) _(PREV) is set 2. The availability derivationprocess for a block as specified in clause 6.4.X [Ed. (BB): Neighbouringblocks availability checking process tbd] is invoked with the location (xCurr, yCurr ) set equal to ( xCb, yCb ) and the neighbouring location (xNbY, yNbY ) set equal to ( xQg − 1, yQg ) as inputs, and the output isassigned to availableA. The variable qP_(Y) _(—) _(A) is derived asfollows: - If one or more of the following conditions are true, qP_(Y)_(—) _(A) is set equal to qP_(Y) _(—) _(PRFV): - availableA is equal toFALSE. - the CTB address ctbAddrA of the CTB containing the luma codingblock covering the luma location ( xQg − 1, yQg ) is not equal toCtbAddrInBs, where ctbAddrA is derived as follows: xTmp = ( xQg − 1 ) >>MinTbLog2SizeY yTmp = yQg >> MinTbLog2SizeY minTbAddrA = MinTbAddrZs[xTmp ][ yTmp ] ctbAddrA = minTbAddrA >> ( 2 * ( CtbLog2SizeY −MinTbLog2SizeY ) ) - Otherwise, qP_(Y) _(—) _(A) is set equal to theluma quantization parameter Qp_(Y) of the coding unit containing theluma coding block covering ( xQg − 1, yQg ). 3. The availabilityderivation process for a block as specified in clause 6.4.X [Ed. (BB):Neighbouring blocks availability checking process tbd] is invoked withthe location ( xCurr, yCurr ) set equal to ( xCb, yCb ) and theneighbouring location ( xNbY, yNbY ) set equal to ( xQg, yQg − 1 ) asinputs, and the output is assigned to availableB. The variable qP_(Y B)is derived as follows: - If one or more of the following conditions aretrue, qP_(Y) _(—) _(B) is set equal to qP_(Y) _(—) _(PREV): - availableBis equal to FALSE. - the CTB address ctbAddrB of the CTB containing theluma coding block covering the luma location ( xQg, yQg − 1 ) is notequal to CtbAddrInBs, where ctbAddrB is derived as follows: xTmp =xQg >> MinTbLog2SizeY yTmp = ( yQg − 1 ) >> MinTbLog2SizeY minTbAddrB =MinTbAddrZs[ xTmp ][ yTmp ] ctbAddrB = minTbAddrB >> ( 2 * (CtbLog2SizeY − MinTbLog2SizeY ) ) (8-922) - Otherwise, qP_(Y) _(—) _(B)is set equal to the luma quantization parameter Qp_(Y) of the codingunit containing the luma coding block covering ( xQg, yQg − 1 ). 4. Thepredicted luma quantization parameter qP_(Y) _(—) _(PRED) is derived asfollows: - If all the following conditions are true, then qP_(Y) _(—)_(PRED) is set equal to the luma quantization parameter Qp_(Y) of thecoding unit containing the luma coding block covering ( xQg, yQg − 1): - availableB is equal to TRUE. - the current quantization group isthe first quantization group in a CTB row within a brick - Otherwise,qP_(Y) _(—) _(PRED) is derived as follows: qP_(Y) _(—) _(PRED) = (qP_(Y) _(—) _(A) + qP_(Y) _(—) _(B) + 1 ) >> 1 The variable Qp_(Y) isderived as follows: QP_(Y) = ( ( qP_(Y) _(—) _(PRED) + CuQpDeltaVal +64 + 2 * QpBdOffset_(Y) )%( 64 + QpBdOffset_(Y) ) ) − Qp BdOffset_(Y)The luma quantization parameter Qp′_(Y) is derived as follows: QP′_(Y) =QP_(Y) + QpBdOffset_(Y) When ChromaArrayType is not equal to 0 andtreeType is equal to SINGLE_TREE or DUAL_TREE_CHROMA, the followingapplies: - When treeType is equal to DUAL_TREE_CHROMA, the variableQp_(Y) is set equal to the luma quantization parameter Qp_(Y) of theluma coding unit that covers the luma location ( xCb + cbWidth / 2,yCb + cbHeight / 2 ). - The variables qP_(Cb), qP_(Cr) and qP_(CbCr) arederived as follows: qPi_(Cb) = Clip3( −QpBdOffset_(C), 69, Qp_(Y) +pps_cb_qp_offset + slice_cb_qp_offset ) qPi_(Cr) = Clip3(−QpBdOffset_(C), 69, Qp_(Y) + pps_cr_qp_offset + slice_cr_qp_offset )qPi_(CbCr) = Clip3( −QpBdOffset_(C), 69, Qp_(Y) +pps_joint_cbcr_qp_offset + slice_joint_c bcr_qp_offset ) - IfChromaArrayType is equal to 1, and Qpc_data_default_flag is equal toFALSE, the variables qP_(Cb), qP_(Cr) and qP_(CbCr) are set equal to thevalue of Qp_(C) as-specified in clause 7.x.x based on the index qPiequal to qPi_(Cb), qPi_(Cr) and qPi_(CbCr), respectively - IfChromaArrayType is equal to 1, and Qpc_data_default_flag is equal toTRUE, the variables qP_(Cb), qP_(Cr) and qP_(CbCr) are specified inTable 8-15 based on the index qPi equal to qPi_(Cb), qPi_(Cr) andqPi_(CbCr), respectively - Otherwise, the variables qP_(Cb), qP_(Cr) andqP_(CbCr) are set equal to Min( qPi, 63 ), based on the index qPi equalto qPi_(Cb), qPi_(Cr) and qPi_(CbCr), respectively. - The chromaquantization parameters for the Cb and Cr components, Qp′_(Cb) andQp′_(Cr), and joint Cb-Cr coding Qp′_(CbCr) are derived as follows:QP′_(Cb) = qP_(Cb) + QpBdOffset_(C) Qp′_(Cr) = qP_(Cr) + QpBdOffset_(C)Qp′_(CbCr) = qP_(CbCr) + QpBdOffset_(C) Table 8-15 - Specification ofQpC as a function of qPi for ChromaArrayType equal to 1 qPi < 30 30 3132 33 34 35 36 37 38 39 40 41 42 43 > 43 Qp_(C) = qPi 29 30 31 32 33 3334 34 35 35 36 36 37 37 = qPi − 6

Referring to Table 28, when ChromaArrayType is 1 andQp_(c)_data_default_flag indicates false (i.e., for example, whenQp_(c)_data_default_flag is 0), the parameters qP_(Cb), qP_(Cr) andqP_(CbCr) may be derived based on signaled user defined information asproposed in the present embodiment. Furthermore, for example, whenChromaArrayType is 1 and Qp_(c)_data_default_flag indicates true (i.e.,for example, when Qp_(c)_data_default_flag is 1), the parametersqP_(Cb), qP_(Cr) and qP_(CbCr) may be derived by a default table basedon the same indices qPi as qPi_(Cb), qPi_(Cr) and qPi_(CbCr),respectively.

Furthermore, this document proposes another embodiment in whichinformation for a quantization parameter is signaled.

For example, in the present embodiment, it is proposed that user definedderivation of chroma quantization is signaled in an SPS as follows. Forexample, the present embodiment proposes user defined chromaquantization (Qp_(C)). For example, the flag of an SPS may representwhether a default table is used for chroma quantization derivation orthe contents of the table for chroma quantization derivation is derivedin information signaled in the SPS.

For example, the present embodiment proposes a scheme for performingchroma quantization as a function of an index qPi by using syntaxelements illustrated in the following table.

TABLE 29 Descriptor Qp_(C)_data( ) {  qPi_min_idx ue(v) qPi_delta_max_idx ue(v)  for ( i = qPi_min_idx; i <= qPiMaxIdx, i++ )  Qp_(C)_qPi_delta_val[ i ] ue(v) }

For example, the syntax element qPi_min_idx may represent a minimum qPiindex used in chroma quantization. A value of qPi_min_idx may be in arange of 0 to 63.

Furthermore, for example, the syntax element qPi_delta_max_idx mayrepresent a delta value between Qpi_min_idx and a maximum qPi index usedin chroma Qp_(c) derivation. A value of qPiMaxIdx may be greater than orequal to qPi_min_idx. A value of qPi_delta_max_idx may be in a range of0 to 63. For example, a maximum index qPiMaxIdx used in Qp_(c)derivation may be derived like Equation 4.

Furthermore, for example, the syntax element Qp_(C)_qPi_delta_val[i] mayrepresent the delta of a Qp_(C) value for an i-th index.

For example, a parameter Qp_(C)Idx[qPi] may be derived as follows.

-   -   When qPi<qPi_min_idx, Qp_(C)Idx[qPi] may be set identically with        qPi.    -   When qPi=qPi_min_idx . . . qPiMaxIdx, Qp_(C)Idx[qPi] may be set        as Qp_(C)_qPi_delta_val[qPi]+Qp_(C)Idx[qPi−1].    -   When qPi>qPiMaxIdx, Qp_(C)Idx[qPi] may be set as        qPi−(qPiMaxIdx−Qp_(C)Idx[qPiMaxIdx]).

Thereafter, the Qp_(C) may be set as the Qp_(C)Idx[qPi].

Furthermore, the flag of an SPS representing whether a default table isused for chroma quantization derivation or whether signaled informationsignaled information is used for chroma quantization derivation, whichis proposed in the present embodiment, may be the same as the followingtable.

TABLE 30 Descriptor seq_parameter_set_rbsp( ) { sps_decoding_parameter_set_id u(4) ...  Qpc_data_default_flag u(1) if(!Qp_(C)_data_default_flag)  Qp_(C)_data( ) ...

For example, the syntax element Qp_(c)_data_default_flag may representwhether a user defined mode is used for the derivation of a quantizationparameter. For example, Qp_(c)_data_default_flag having a value of 0 mayrepresent that a user defined mode is used for the derivation of aquantization parameter. Furthermore, for example,Qp_(c)_data_default_flag having a value of 1 may represent that adefault table is used for the derivation of a quantization parameter.The default table may be the same as Table 7. Furthermore, ifQp_(c)_data_default_flag is not present, the Qp_(c)_data_default_flagmay be inferred as 1.

For example, according to the present embodiment, if a process ofderiving a quantization parameter is written in a standard format, theprocess may be represented as in the following table.

TABLE 31 8.7.1 Derivation process for quantization parameters Inputs tothis process are: -a luma location ( xCb, yCb ) specifying the top-leftluma sample of the current coding block relative to the top-left lumasample of the current picture, -a variable cbWidth specifying the widthof the current coding block in luma samples, -a variable cbHeightspecifying the height of the current coding block in luma samples, -avariable treeType specifying whether a single tree (SINGLE_TREE) or adual tree is used to partition the CTUs and, when a dual tree is used,whether the luma (DUAL_TREE_LUMA) or chroma components(DLAL_TREE_CHROMA) are currently processed. In this process, the lumaquantization parameter Qp′_(Y) and the chroma quantization parametersQp′_(Cb) and Qp′_(Cr) are derived. The luma location ( xQg, yQg ),specifies the top-left luma sample of the current quantization grouprelative to the top left luma sample of the current picture. Thehorizontal and vertical positions xQg and yQg are set equal toCuQgTopLeftX and CuQgTopLeftY, respectively. NOTE - : The currentquantization group is a rectangluar region inside a coding tree blockthat shares the same qP_(Y) _(—) _(PRED). Its width and height are equalto the width and height of the coding tree node of which the top-leftluma sample position is assigned to the variables CuQgTopLeftX andCuQgTopLeftY. When treeType is equal to SINGLE_TREE or DUAL_TREE_LUMA,the predicted luma quantization parameter qP_(Y) _(—) _(PRED) is derivedby the following ordered steps: 1.The variable qP_(Y) _(—) _(PREV) isderived as follows: - If one or more of the following conditions aretrue, qP_(Y) _(—) _(PREV) is set equal to SliceQp_(Y): - The currentquantization group is the first quantization group in a slice. - Thecurrent quantization group is the first quantization group in a brick. -Otherwise, qP_(Y) _(—) _(PREV) is set 2. The availability derivationprocess for a block as specified in clause 6.4.X [Ed. (BB): Neighbouringblocks availability checking process tbd] is invoked with the location (xCurr, yCurr ) set equal to ( xCb, yCb ) and the neighbouring location (xNbY, yNbY ) set equal to ( xQg − 1, yQg ) as inputs, and the output isassigned to availableA. The variable qP_(Y) _(—) _(A) is derived asfollows: - If one or more of the following conditions are true, qP_(Y)_(—) _(A) is set equal to qP_(Y) _(—) _(PREV): - availableA is equal toFALSE. - the CTB address ctbAddrA of the CTB containing the luma codingblock covering the luma location ( xQg − 1, yQg ) is not equal toCtbAddrInBs, where ctbAddrA is derived as follows: xTmp = ( xQg − 1 ) >>MinTbLog2SizeY yTmp = yQg >> MinTbLog2SizeY minTbAddrA = MinTbAddrZs[xTmp ][ yTmp ] ctbAddrA = minTbAddrA >> ( 2 * ( CtbLog2SizeY −MinTbLog2SizeY ) ) - Otherwise, qP_(Y) _(—) _(A) is set equal to theluma quantization parameter Qp_(Y) of the coding unit containing theluma coding block covering ( xQg − 1, yQg ). 3. The availabilityderivation process for a block as specified in clause 6.4.X [Ed. (BB):Neighbouring blocks availability checking process tbd] is invoked withthe location ( xCurr, yCurr ) set equal to ( xCb, yCb ) and theneighbouring location ( xNbY, yNbY ) set equal to ( xQg, yQg − 1 ) asinputs, and the output is assigned to availableB. The variable qP_(Y B)is derived as follows: - If one or more of the following conditions aretrue, qP_(Y) _(—) _(B) is set equal to qP_(Y) _(—) _(PREV): - availableBis equal to FALSE. - the CTB address ctbAddrB of the CTB containing theluma coding block covering the luma location ( xQg, yQg − 1 ) is notequal to CtbAddrInBs, where ctbAddrB is derived as follows: xTmp =xQg >> MinTbLog2SizeY yTmp = ( yQg − 1 ) >> MinTbLog2SizeY minTbAddrB =MinTbAddrZs[ xTmp ][ yTmp ] ctbAddrB = minTbAddrB >> ( 2 * (CtbLog2SizeY − MinTbLog2SizeY ) ) (8-922) - Otherwise, qP_(Y) _(—) _(B)is set equal to the luma quantization parameter Qp_(Y) of the codingunit containing the luma coding block covering ( xQg, yQg − 1 ). 4. Thepredicted luma quantization parameter qP_(Y) _(—) _(PRED) is derived asfollows: - If all the following conditions are true, then qP_(Y) _(—)_(PRED) is set equal to the luma quantization parameter Qp_(Y) of thecoding unit containing the luma coding block covering ( xQg, yQg − 1): - availableB is equal to TRUE. - the current quantization group isthe first quantization group in a CTB row within a brick - Otherwise,qP_(Y) _(—) _(PRED) is derived as follows: qP_(Y) _(—) _(PRED) = (qP_(Y) _(—) _(A) + qP_(Y) _(—) _(B) + 1 ) >> 1 The variable Qp_(Y) isderived as follows: QP_(Y) = ( ( qP_(Y PRED) + CuQpDeltaVal + 64 + 2 *QpBdOffset_(Y))%( 64 + QpBdOffset_(Y)) ) − Qp BdOffset_(Y) The lumaquantization parameter Qp′_(Y) is derived as follows: Qp′_(Y) = QP_(Y) +QpBdOffset_(Y) When ChromaArrayType is not equal to 0 and treeType isequal to SINGLE_TREE or DUAL_TREE_CHROMA, the following applies: - WhentreeType is equal to DUAL_TREE_CHROMA, the variable Qp_(Y) is set equalto the luma quantization parameter Qp_(Y) of the luma coding unit thatcovers the luma location ( xCb + cbWidth / 2, yCb + cbHeight / 2 ). -The variables qP_(Cb), qP_(Cr) and qP_(CbCr) are derived as follows:qPi_(Cb) = Clip3( −QpBdOffset_(C), 69, Qp_(Y) + pps_cb_qp_offset +slice_cb_qp_offset ) qPi_(Cr) = Clip3( −QpBdOffset_(C), 69, Qp_(Y) +pps_cr_qp_offset + slice_cr_qp_offset ) qPi_(CbCr) = Clip3(−QpBdOffset_(C), 69, Qp_(Y) + pps_joint_cbcr_qp_offset + slice_joint_cbcr_qp_offset ) - If ChromaArrayType is equal to 1, andQpc_data_default_flag is equal to FALSE, the variables qP_(Cb), qP_(Cr)and qP_(CbCr) are set equal to the value of Qp_(C) as-specified inclause 7.x.x based on the index qPi equal to qPi_(Cb), qPi_(Cr) andqPi_(CbCr), respectively. - If ChromaArrayType is equal to 1, andQpc_data_default_flag is equal to TRUE, the variables qP_(Cb), qP_(Cr)and qP_(CbCr) are specified in Table 8-15 based on the index qPi equalto qPi_(Cb), qPi_(Cr) and qPi_(CbCr), respectively. - Otherwise, thevariables qP_(Cb), qP_(Cr) and qP_(CbCr) are set equal to Min( qPi, 63), based on the index qPi equal to qPi_(Cb), qPi_(Cr) and qPi_(CbCr),respectively. - The chroma quantization parameters for the Cb and Crcomponents, Qp′_(Cb) and Qp′_(Cr), and joint Cb-Cr coding Qp′_(CbCr) arederived as follows: Qp′_(Cb) = qP_(Cb) + QpBdOffset_(C) Qp′_(Cr) =qP_(Cr) + QpBdOffset_(C) Qp′_(CbCr) = qP_(CbCr) + QpBdOffset_(C) Table8-15 - Specification of QpC as a function of qPi for ChromaArrayTypeequal to 1 qPi < 30 30 31 32 33 34 35 36 37 38 39 40 41 42 43 > 43Qp_(C) = qPi 29 30 31 32 33 33 34 34 35 35 36 36 37 37 = qPi − 6

Referring to Table 31, when ChromaArrayType is 1 andQp_(c)_data_default_flag indicates false (i.e., for example, whenQp_(c)_data_default_flag is 0), the parameters qP_(Cb), qP_(Cr) andqP_(CbCr) may be derived based on signaled user defined information asproposed in the present embodiment. Furthermore, for example, whenChromaArrayType is 1 and Qp_(c)_data_default_flag indicates true (i.e.,for example, when Qp_(c)_data_default_flag is 1), the parametersqP_(Cb), qP_(Cr) and qP_(CbCr) may be derived by a default table basedon the same indices qPi as qPi_(Cb), qPi_(Cr) and qPi_(CbCr),respectively.

Furthermore, this document proposes another embodiment in whichinformation for a quantization parameter is signaled.

For example, the present embodiment proposes adding a function forsignaling a chroma quantization parameter Qp_(C) as a function of anindex qPi. For example, there may be proposed a scheme for signalingsyntax elements for a user defined table for quantization parameterderivation in a PPS. Accordingly, flexibility regarding changing a userdefined table and a default table in each picture that refers to a PPScan be provided.

Syntax elements for a signaled user defined table in a PPS proposed inthe present embodiment may be the same as the following table.

TABLE 32 Descriptor Qp_(C)_data( ) {  qPi_min_idx ue(v) qPi_delta_max_idx ue(v)  for ( i = qPi_min_idx; i <= qPiMaxIdx; i++ )  Qp_(C)_qPi_delta_val[ i ] ue(v) }

For example, the syntax element qPi_min_idx may represent a minimum qPiindex used in chroma quantization. A value of qPi_min_idx may be in arange of 0 to 63.

Furthermore, for example, the syntax element qPi_delta_max_idx mayrepresent a delta value between Qpi_min_idx and a maximum qPi index usedin chroma Qp_(c) derivation. A value of qPiMaxIdx may be greater than orequal to qPi_min_idx. A value of qPi_delta_max_idx may be in a range of0 to 63. For example, a maximum index qPiMaxIdx used in Qp_(c)derivation may be derived like Equation 4.

Furthermore, for example, the syntax element Qp_(C)_qPi_delta_val[i] mayrepresent the delta of a Qp_(C) value for an i-th index.

For example, a parameter Qp_(C)Idx[qPi] may be derived as follows.

-   -   When qPi<qPi_min_idx, Qp_(C)Idx[qPi] may be set identically with        qPi.    -   When qPi=qPi_min_idx . . . qPiMaxIdx, Qp_(C)Idx[qPi] may be set        as Qp_(C)_qPi_delta_val[qPi]+Qp_(C)Idx[qPi−1].    -   When qPi>qPiMaxIdx, Qp_(C)Idx[qPi] may be set as        qPi−(qPiMaxIdx−Qp_(C)Idx[qPiMaxIdx]).

Thereafter, the Qp_(C) may be set as the Qp_(C)Idx[qPi].

Furthermore, the flag of SPS representing whether a default table isused for chroma quantization derivation or whether signaled informationis used for chroma quantization derivation, which is proposed in thepresent embodiment, may be the same as the following table.

TABLE 33 Descriptor seq_parameter_set_rbsp( ) { sps_decoding_parameter_set_id u(4) ...  Qpc_data_default_flag u(1) if(!Qp_(C)_data_default_flag)  Qp_(C)_data( ) ...

For example, the syntax element Qp_(c)_data_default_flag may representwhether a user defined mode is used for the derivation of a quantizationparameter. For example, Qp_(c)_data_default_flag having a value of 0 mayrepresent that a user defined mode is used for the derivation of aquantization parameter. That is, for example, Qp_(c)_data_default_flaghaving a value of 0 may represent that chroma quantization parameterdata Qp_(c)_data( ) is used. When the Qp_(c)_data_default_flag is 0, thechroma quantization parameter data Qp_(c)_data( ) may be signaled.Furthermore, for example, Qp_(c)_data_default_flag having a value of 1may represent that a default table is used for the derivation of aquantization parameter. The default table may be the same as Table 7.Furthermore, if Qp_(c)_data_default_flag is not present, theQp_(c)_data_default_flag may be inferred as 1.

For example, in the present embodiment, if a process of deriving aquantization parameter is written in a standard format, the process maybe represented as in the following table.

TABLE 34 8.7.1 Derivation process for quantization parameters Inputs tothis process are: -a luma location ( xCb, yCb ) specifying the top-leftluma sample of the current coding block relative to the top-left lumasample of the current picture, -a variable cbWidth specifying the widthof the current coding block in luma samples, -a variable cbHeightspecifying the height of the current coding block in luma samples, -avariable treeType specifying whether a single tree (SINGLE_TREE) or adual tree is used to partition the CTUs and, when a dual tree is used,whether the luma (DUAL_TREE_LUMA) or chroma components(DLAL_TREE_CHROMA) are currently processed. In this process, the lumaquantization parameter Qp′_(Y) and the chroma quantization parametersQp′_(Cb) and Qp′_(Cr) are derived. The luma location ( xQg, yQg ),specifies the top-left luma sample of the current quantization grouprelative to the top left luma sample of the current picture. Thehorizontal and vertical positions xQg and yQg are set equal toCuQgTopLeftX and CuQgTopLeftY, respectively. NOTE - : The currentquantization group is a rectangluar region inside a coding tree blockthat shares the same qP_(Y) _(—) _(PRED). Its width and height are equalto the width and height of the coding tree node of which the top-leftluma sample position is assigned to the variables CuQgTopLeftX andCuQgTopLeftY. When treeType is equal to SINGLE_TREE or DUAL_TREE_LUMA,the predicted luma quantization parameter qP_(Y) _(—) _(PRED) is derivedby the following ordered steps: 1.The variable qP_(Y) _(—) _(PREV) isderived as follows: - If one or more of the following conditions aretrue, qP_(Y) _(—) _(PREV) is set equal to SliceQp_(Y): - The currentquantization group is the first quantization group in a slice. - Thecurrent quantization group is the first quantization group in a brick. -Otherwise, qP_(Y) _(—) _(PREV) is set 2. The availability derivationprocess for a block as specified in clause 6.4.X [Ed. (BB): Neighbouringblocks availability checking process tbd] is invoked with the location (xCurr, yCurr ) set equal to ( xCb, yCb ) and the neighbouring location (xNbY, yNbY ) set equal to ( xQg − 1, yQg ) as inputs, and the output isassigned to availableA. The variable qP_(Y) _(—) _(A) is derived asfollows: - If one or more of the following conditions are true, qP_(Y A)is set equal to qP_(Y PREV): - availableA is equal to FALSE. - the CTBaddress ctbAddrA of the CTB containing the luma coding block coveringthe luma location ( xQg − 1, yQg ) is not equal to CtbAddrInBs, wherectbAddrA is derived as follows: xTmp = ( xQg − 1 ) >> MinTbLog2SizeYyTmp = yQg >> MinTbLog2SizeY minTbAddrA = MinTbAddrZs[ xTmp ][ yTmp ]ctbAddrA = minTbAddrA >> ( 2 * ( CtbLog2SizeY − MinTbLog2SizeY ) ) -Otherwise, qP_(Y) _(—) _(A) is set equal to the luma quantizationparameter Qp_(Y) of the coding unit containing the luma coding blockcovering ( xQg − 1, yQg ). 3. The availability derivation process for ablock as specified in clause 6.4.X [Ed. (BB): Neighbouring blocksavailability checking process tbd] is invoked with the location ( xCurr,yCurr ) set equal to ( xCb, yCb ) and the neighbouring location ( xNbY,yNbY ) set equal to ( xQg, yQg − 1 ) as inputs, and the output isassigned to availableB. The variable qP_(Y) _(—) _(B) is derived asfollows: - If one or more of the following conditions are true, qP_(Y B)is set equal to qP_(Y PREV): - availableB is equal to FALSE. - the CTBaddress ctbAddrB of the CTB containing the luma coding block coveringthe luma location ( xQg, yQg − 1 ) is not equal to CtbAddrInBs, wherectbAddrB is derived as follows: xTmp = xQg >> MinTbLog2SizeY yTmp = (yQg − 1 ) >> MinTbLog2SizeY minTbAddrB = MinTbAddrZs[ xTmp ][ yTmp ]ctbAddrB = minTbAddrB >> ( 2 * ( CtbLog2SizeY − MinTbLog2SizeY ) )(8-922) - Otherwise, qP_(Y) _(—) _(B) is set equal to the lumaquantization parameter Qp_(Y) of the coding unit containing the lumacoding block covering ( xQg, yQg − 1 ). 4. The predicted lumaquantization parameter qP_(Y) _(—) _(PRED) is derived as follows: - Ifall the following conditions are true, then qP_(Y) _(—) _(PRED) is setequal to the luma quantization parameter Qp_(Y) of the coding unitcontaining the luma coding block covering ( xQg, yQg − 1 ): - availableBis equal to TRUE. - the current quantization group is the firstquantization group in a CTB row within a brick - Otherwise, qP_(Y) _(—)_(PRED) is derived as follows: qP_(Y PRED) = ( qP_(Y A) + qP_(Y B) + 1) >> 1 The variable Qp_(Y) is derived as follows: QP_(Y) = ( ( qP_(Y)_(—) _(PRED) + CuQpDeltaVal + 64 + 2 * QpBdOffset_(Y) )%( 64 +QpBdOffset_(Y)) ) − Qp BdOffset_(Y) The luma quantization parameterQp′_(Y) is derived as follows: Qp′_(Y) = QP_(Y) + QpBdOffset_(Y) WhenChromaArrayType is not equal to 0 and treeType is equal to SINGLE_TREEor DUAL_TREE_CHROMA, the following applies: - When treeType is equal toDUAL_TREE_CHROMA, the variable Qp_(Y) is set equal to the lumaquantization parameter Qp_(Y) of the luma coding unit that covers theluma location ( xCb + cbWidth / 2, yCb + cbHeight / 2 ). - The variablesqP_(Cb), qP_(Cr) and qP_(CbCr) are derived as follows: qPi_(Cb) = Clip3(−QpBdOffset_(C), 69, Qp_(Y) + pps_cb_qp_offset + slice_cb_qp_offset )qPi_(Cr) = Clip3( −QpBdOffset_(C), 69, Qp_(Y) + pps_cr_qp_offset +slice_cr_qp_offset ) qPi_(CbCr) = Clip3( −QpBdOffset_(C), 69, Qp_(Y) +pps_joint_cbcr_qp_offset + slice_joint_c bcr_qp_offset ) - IfChromaArrayType is equal to 1, and Qpc_data_default_flag is equal toFALSE, the variables qP_(Cb), qP_(Cr) and qP_(CbCr) are set equal to thevalue of Qp_(C) as-specified in clause 7.x.x based on the index qPiequal to qPi_(Cb), qPi_(Cr) and qPi_(CbCr), respectively. - IfChromaArrayType is equal to 1, and Qpc_data_default_flag is equal toTRUE, the variables qP_(Cb), qP_(Cr) and qP_(CbCr) are specified inTable 8-15 based on the index qPi equal to qPi_(Cb), qPi_(Cr) andqPi_(CbCr), respectively. - Otherwise, the variables qP_(Cb), qP_(Cr)and qP_(CbCr) are set equal to Min( qPi, 63 ), based on the index qPiequal to qPi_(Cb), qPi_(Cr) and qPi_(CbCr), respectively. - The chromaquantization parameters for the Cb and Cr components, Qp′_(Cb) andQp′_(Cr), and joint Cb-Cr coding Qp′_(CbCr) are derived as follows:Qp′_(Cb) = qP_(Cb) + QpBdOffset_(C) Qp′_(Cr) = qP_(Cr) + QpBdOffset_(C)QP′_(CbCr) = qP_(CbCr) + QpBdOffset_(C) Table 8-15 - Specification ofQpC as a function of qPi for ChromaArrayType equal to 1 qPi < 30 30 3132 33 34 35 36 37 38 39 40 41 42 43 > 43 Qp_(C) = qPi 29 30 31 32 33 3334 34 35 35 36 36 37 37 = qPi − 6

Referring to Table 34, when ChromaArrayType is 1 andQp_(c)_data_default_flag indicates false (i.e., for example, whenQp_(c)_data_default_flag is 0), the parameters qP_(Cb), qP_(Cr) andqP_(CbCr) may be derived based on signaled user defined information asproposed in the present embodiment. Furthermore, for example,ChromaArrayType is 1 and Qp_(c)_data_default_flag indicates true (i.e.,for example, when Qp_(c)_data_default_flag is 1), the parametersqP_(Cb), qP_(Cr) and qP_(CbCr) may be derived by a default table basedon the same indices qPi as qPi_(Cb), qPi_(Cr) and qPi_(CbCr),respectively.

Furthermore, this document proposes another embodiment in whichinformation for a quantization parameter is signaled.

For example, the present embodiment proposes a common mode in which achroma quantization parameter Qp_(C) is derived and signaled.

Chroma quantization parameter data Qp_(c)_data( ) for a chromaquantization parameter proposed in the present embodiment may besignaled as in the following table.

TABLE 35 Descriptor Qp_(C)_data( ) {  qPi_min_idx ue(v) qPi_delta_max_idx ue(v)  for ( i = qPi_min_idx; i <= qPiMaxIdx; i++ )  Qp_(C)_qPi_delta_val[ i ] ue(v) }

For example, the syntax element qPi_min_idx may represent a minimum qPiindex used in chroma quantization. A value of qPi_min_idx may be in arange of 0 to 63.

Furthermore, for example, the syntax element qPi_delta_max_idx mayrepresent a delta value between Qpi_min_idx and a maximum qPi index usedin chroma Qp_(c) derivation. A value of qPiMaxIdx may be greater than orequal to qPi_min_idx. A value of qPi_delta_max_idx may be in a range of0 to 63. For example, a maximum index qPiMaxIdx used in Qp_(c)derivation may be derived like Equation 4.

Furthermore, for example, the syntax element Qp_(C)_qPi_delta_val[i] mayrepresent the delta of a Qp_(C) value for an i-th index.

For example, a parameter Qp_(C)Idx[qPi] may be derived as follows.

-   -   When qPi<qPi_min_idx, Qp_(C)Idx[qPi] may be set identically with        qPi.    -   When qPi=qPi_min_idx . . . qPiMaxIdx, Qp_(C)Idx[qPi] may be set        as Qp_(C)_qPi_delta_val[qPi]+Qp_(C)Idx[qPi−1].    -   When qPi>qPiMaxIdx, Qp_(C)Idx[qPi] may be set as        qPi−(qPiMaxIdx−Qp_(C)Idx[qPiMaxIdx]).

Thereafter, the Qp_(C) may be set as the Qp_(C)Idx[qPi].

Furthermore, the present embodiment proposes a scheme for signaling aflag representing whether a default table is used for chromaquantization derivation or whether signaled information is used forchroma quantization derivation. The flag may be signaled through a highlevel syntax, such as a sequence parameter set (SPS), or a pictureparameter set (PPS). The flag signaled through a high level syntax maybe the same as the following table.

TABLE 36 Descriptor high_level_parameter_set( ) { ... Qpc_data_default_flag u(1)  if(!Qp_(C)_data_default_flag)  Qp_(C)_data() ...

For example, the syntax element Qp_(c)_data_default_flag may representwhether a user defined mode is used for the derivation of a quantizationparameter. For example, Qp_(c)_data_default_flag having a value of 0 mayrepresent that a user defined mode is used for the derivation of aquantization parameter. That is, for example, Qp_(c)_data_default_flaghaving a value of 0 may represent chroma quantization parameter dataQp_(c)_data( ) is used. When the Qp_(c)_data_default_flag is 0, thechroma quantization parameter data Qp_(c)_data( ) may be signaled.Furthermore, for example, Qp_(c)_data_default_flag having a value of 1may represent that a default table is used for the derivation of aquantization parameter. The default table may be the same as Table 7.Furthermore, if Qp_(c)_data_default_flag is not present, theQp_(c)_data_default_flag may be inferred as 1.

For example, in the present embodiment, if a process of deriving aquantization parameter is written in a standard format, the process maybe represented as in the following table.

TABLE 37 8.7.1 Derivation process for quantization parameters Inputs tothis process are: -a luma location ( xCb, yCb ) specifying the top-leftluma sample of the current coding block relative to the top-left lumasample of the current picture, -a variable cbWidth specifying the widthof the current coding block in luma samples, -a variable cbHeightspecifying the height of the current coding block in luma samples, -avariable treeType specifying whether a single tree (SINGLE_TREE) or adual tree is used to partition the CTUs and, when a dual tree is used,whether the luma (DUAL_TREE_LUMA) or chroma components(DUAL_TREE_CHROMA) are currently processed. In this process, the lumaquantization parameter Qp′_(Y) and the chroma quantization parametersQp′_(Cb) and Qp′_(Cr) are derived. The luma location ( xQg, yQg ),specifies the top-left luma sample of the current quantization grouprelative to the top left luma sample of the current picture. Thehorizontal and vertical positions xQg and yQg are set equal toCuQgTopLeftX and CuQgTopLeftY, respectively. NOTE - : The currentquantization group is a rectangluar region inside a coding tree blockthat shares the same qP_(Y) _(—) _(PRED). Its width and height are equalto the width and height of the coding tree node of which the top-leftluma sample position is assigned to the variables CuQgTopLeftX andCuQgTopLeftY. When treeType is equal to SINGLE_TREE or DUAL_TREE_LUMA,the predicted luma quantization parameter qP_(Y) _(—) _(PRED) is derivedby the following ordered steps: 1.The variable qP_(Y) _(—) _(PREV) isderived as follows: - If one or more of the following conditions aretrue, qP_(Y) _(—) _(PREV) is set equal to SliceQp_(Y): - The currentquantization group is the first quantization group in a slice. - Thecurrent quantization group is the first quantization group in a brick. -Otherwise, qP_(Y) _(—) _(PREV) is set 2. The availability derivationprocess for a block as specified in clause 6.4.X [Ed. (BB): Neighbouringblocks availability checking process tbd] is invoked with the location (xCurr, yCurr ) set equal to ( xCb, yCb ) and the neighbouring location (xNbY, yNbY ) set equal to ( xQg − 1, yQg ) as inputs, and the output isassigned to availableA. The variable qP_(Y) _(—) _(A) is derived asfollows: - If one or more of the following conditions are true, qP_(Y)_(—) _(A) is set equal to qP_(Y) _(—) _(PREV): - availableA is equal toFALSE. - the CTB address ctbAddrA of the CTB containing the luma codingblock covering the luma location ( xQg − 1, yQg ) is not equal toCtbAddrInBs, where ctbAddrA is derived as follows: xTmp = ( xQg − 1 ) >>MinTbLog2SizeY yTmp = yQg >> MinTbLog2SizeY minTbAddrA = MinTbAddrZs[xTmp ][ yTmp ] ctbAddrA = minTbAddrA >> ( 2 * ( CtbLog2SizeY −MinTbLog2SizeY ) ) - Otherwise, qP_(Y) _(—) _(A) is set equal to theluma quantization parameter Qp_(Y) of the coding unit containing theluma coding block covering ( xQg − 1, yQg ). 3. The availabilityderivation process for a block as specified in clause 6.4.X [Ed. (BB):Neighbouring blocks availability checking process tbd] is invoked withthe location ( xCurr, yCurr ) set equal to ( xCb, yCb ) and theneighbouring location ( xNbY, yNbY ) set equal to ( xQg, yQg − 1 ) asinputs, and the output is assigned to availableB. The variable qP_(Y B)is derived as follows: - If one or more of the following conditions aretrue, qP_(Y) _(—) _(B) is set equal to qP_(Y) _(—) _(PREV): - availableBis equal to FALSE. - the CTB address ctbAddrB of the CTB containing theluma coding block covering the luma location ( xQg, yQg − 1 ) is notequal to CtbAddrInBs, where ctbAddrB is derived as follows: xTmp =xQg >> MinTbLog2SizeY yTmp = ( yQg − 1 ) >> MinTbLog2SizeY minTbAddrB =MinTbAddrZs[ xTmp ][ yTmp ] ctbAddrB = minTbAddrB >> ( 2 * (CtbLog2SizeY − MinTbLog2SizeY ) ) (8-922) - Otherwise, qP_(Y B) is setequal to the luma quantization parameter Qp_(Y) of the coding unitcontaining the luma coding block covering ( xQg, yQg − 1 ). 4. Thepredicted luma quantization parameter qP_(Y) _(—) _(PRED) is derived asfollows: - If all the following conditions are true, then qP_(Y PRED) isset equal to the luma quantization parameter Qp_(Y) of the coding unitcontaining the luma coding block covering ( xQg, yQg − 1 ): - availableBis equal to TRUE. - the current quantization group is the firstquantization group in a CTB row within a brick - Otherwise, qP_(Y PRED)is derived as follows: qP_(Y) _(—) _(PRED) = ( qP_(Y) _(—) _(A) + qP_(Y)_(—) _(B) + 1 ) >> 1 The variable Qp_(Y) is derived as follows: Qp_(Y) =( ( qP_(Y) _(—) _(PRED) + CuQpDeltaVal + 64 + 2 * QpBdOffset_(Y))%( 64 +QpBdOffset_(Y)) ) − Qp BdOffset_(Y) The luma quantization parameterQp′_(Y) is derived as follows: Qp′_(Y) = QP_(Y) + QpBdOffset_(Y) WhenChromaArrayType is not equal to 0 and treeType is equal to SINGLE_TREEor DUAL_TREE_CHROMA, the following applies: - When treeType is equal toDUAL_TREE_CHROMA, the variable Qp_(Y) is set equal to the lumaquantization parameter Qp_(Y) of the luma coding unit that covers theluma location ( xCb + cbWidth / 2, yCb + cbHeight / 2 ). - The variablesqP_(Cb), qP_(Cr) and qP_(CbCr) are derived as follows: qPi_(Cb) = Clip3(−QpBdOffset_(C), 69, Qp_(Y) + pps_cb_qp_offset + slice_cb_qp_offset )qPi_(Cr) = Clip3( −QpBdOffset_(C), 69, Qp_(Y) + pps_cr_qp_offset +slice_cr_qp_offset ) qPi_(CbCr) = Clip3( −QpBdOffset_(C), 69, QpY +pps_joint_cbcr_qp_offset + slice_joint_c bcr_qp_offset ) - IfChromaArrayType is equal to 1, and Qpc_data_default_flag is equal toFALSE, the variables qP_(Cb), qP_(Cr) and qP_(CbCr) are set equal to thevalue of Qp_(C) as-specified in clause 7.x.x based on the index qPiequal to qPi_(Cb), qPi_(Cr) and qPi_(CbCr), respectively. - IfChromaArrayType is equal to 1, and Qpc_data_default_flag is equal toTRUE, the variables qP_(Cb), qP_(Cr) and qP_(CbCr) are specified inTable 8-15 based on the index qPi equal to qPi_(Cb), qPi_(Cr) andqPi_(CbCr), respectively. - Otherwise, the variables qP_(Cb), qP_(Cr)and qP_(CbCr) are set equal to Min( qPi, 63 ), based on the index qPiequal to qPi_(Cb), qPi_(Cr) and qPi_(CbCr), respectively. - The chromaquantization parameters for the Cb and Cr components, Qp′_(Cb) andQp′_(Cr), and joint Cb-Cr coding Qp′_(CbCr) are derived as follows:QP′_(Cb) = qP_(Cb) + QpBdOffset_(C) Qp′_(Cr) = qP_(Cr) + QpBdOffset_(C)Qp′_(CbCr) = qP_(CbCr) + QpBdOffset_(C) Table 8-15 - Specification ofQpC as a function of qPi for ChromaArrayType equal to 1 qPi < 30 30 3132 33 34 35 36 37 38 39 40 41 42 43 > 43 Qp_(C) = qPi 29 30 31 32 33 3334 34 35 35 36 36 37 37 = qPi − 6

Referring to Table 37, when ChromaArrayType is 1 andQp_(c)_data_default_flag indicates false (i.e., for example, whenQp_(c)_data_default_flag is 0), the parameters qP_(Cb), qP_(Cr) andqP_(CbCr) may be derived based on signaled user defined information asproposed in the present embodiment. Furthermore, for example, whenChromaArrayType is 1 and Qp_(c)_data_default_flag indicates true (i.e.,for example, when Qp_(c)_data_default_flag is 1), the parametersqP_(Cb), qP_(Cr) and qP_(CbCr) may be derived by a default table basedon the same indices qPi as qPi_(Cb), qPi_(Cr) and qPi_(CbCr),respectively.

Furthermore, this document proposes another embodiment in whichinformation for a quantization parameter is signaled.

For example, the present embodiment proposes a scheme for deriving achroma quantization parameter Qp_(C) table without an offset. Thepresent embodiment may be proposed to be used along with an APS or to beused independently. For example, a syntax structure of APS integratedwith chroma quantization data may be the same as the following table.

TABLE 38 Descriptor Qp_(C)_data ( ) {  qPi_min_idx ue(v) qPi_delta_max_idx ue(v)  for ( i = qPi_min_idx; i <= qPiMaxIdx; i++ )  Qp_(C)_qPi_delta_val[ i ] ue(v) }

For example, the syntax element qPi_min_idx may represent a minimum qPiindex used in chroma quantization. A value of qPi_min_idx may be in arange of 0 to 63.

Furthermore, for example, the syntax element qPi_delta_max_idx mayrepresent a delta value between Qpi_min_idx and a maximum qPi index usedin chroma Qp_(c) derivation. A value of qPiMaxIdx may be greater than orequal to qPi_min_idx. A value of qPi_delta_max_idx may be in a range of0 to 63. For example, a maximum index qPiMaxIdx used in Qp_(c)derivation may be derived like Equation 4.

Furthermore, for example, the syntax element Qp_(C)_qPi_delta_val[i] mayrepresent a difference between Qp_(C) values for an i-th index. Thedifference may also be called a delta.

For example, a parameter Qp_(C)Idx[qPi] may be derived as follows. Inthis case, the qPi may be 0 to 63.

-   -   When qPi<qPi_min_idx, Qp_(C)Idx[qPi] may be set identically with        qPi.    -   qPi=When qPi_min_idx . . . qPiMaxIdx, Qp_(C)Idx[qPi] may be set        as Qp_(C)_qPi_delta_val[qPi]+Qp_(C)Idx[qPi−1].    -   When qPi>qPiMaxIdx, Qp_(C)Idx[qPi] may be set as        qPi−(qPiMaxIdx−Qp_(C)Idx[qPiMaxIdx]).

Thereafter, the Qp_(C) may be set as the Qp_(C)Idx[qPi].

Furthermore, this document proposes another embodiment in whichinformation for a quantization parameter is signaled.

For example, the present embodiment proposes, as an example, a scheme inwhich a delta (or a difference) between continuous Qp_(C) values islimited to 1.

For example, the present embodiment proposes a scheme in which userdefined chroma quantization (Qp_(C)) is additionally included in theexisting image/video standard. For example, the flag of a sequenceparameter set (SPS) proposed in the present embodiment may representwhether the existing default table is used for chroma quantizationparameter derivation or whether the contents of a table is derived basedon information signaled in an SPS. A scheme suitable for a coded imagemay be selected by accommodating user defined chroma quantizationaccording to the present embodiment, and coding efficiency can beimproved.

For example, the present embodiment proposes adding a function forsignaling chroma quantization Qp_(C) as a function of an index qPi byusing syntax elements as in the following table.

TABLE 39 Descriptor Qp_(C)_data( ) {  qPi_min_idx ue(v) qPi_delta_max_idx ue(v)  for ( i = qPi_min_idx; i <= qPiMaxIdx; i++ )  QP_(C)_qPi_flag[ i ] u(1) }

For example, the syntax element qPi_min_idx may represent a minimum qPiindex used in chroma quantization. A value of qPi_min_idx may be in arange of 1 to 63.

Furthermore, for example, the syntax element qPi_delta_max_idx mayrepresent a delta value between Qpi_min_idx and a maximum qPi index usedin chroma Qp_(c) derivation. A value of qPiMaxIdx may be greater than orequal to qPi_min_idx. A value of qPi_delta_max_idx may be in a range of1 to 63. For example, a maximum index qPiMaxIdx used in Qp_(c)derivation may be derived like Equation 4.

Furthermore, for example, the syntax element QpC_qPi_flag[i] mayrepresent whether a Qp_(C) value is increased by 1. That is, forexample, the syntax element QpC_qPi_flag[i] may represent whether ani-th Qp_(C) value has increased by 1 compared to an (i−1)-th Qp_(C)value. For example, QpC_qPi_flag[i] having a value of 1 may represent aQp_(C) value is increased by 1. QpC_qPi_flag[i] having a value of 0 mayrepresent that a Qp_(C) value is not increased.

For example, a parameter Qp_(C)Idx[qPi] may be derived as follows. Inthis case, the qPi may be 0 to 63.

-   -   When qPi<qPi_min_idx, Qp_(C)Idx[qPi] may be set identically with        qPi.    -   When qPi=qPi_min_idx . . . qPiMaxIdx, Qp_(C)Idx[qPi] may be set        as Qp_(C)_qPi_flag[qPi]+Qp_(C)Idx[qPi−1].    -   When qPi>qPiMaxIdx, Qp_(C)Idx[qPi] may be set as        qPi−(qPiMaxIdx−Qp_(C)Idx[qPiMaxIdx]).

Thereafter, the Qp_(C) may be set as the Qp_(C)Idx[qPi].

Furthermore, the present embodiment proposes a scheme for signaling aflag representing whether a default table is used for chromaquantization derivation or whether signaled information is used forchroma quantization derivation. The flag may be signaled through a highlevel syntax, such as a sequence parameter set (SPS), or a pictureparameter set (PPS). The flag signaled through a high level syntax maybe the same as the following table.

TABLE 40 Descriptor seq_parameter_set_rbsp( ) { sps_decoding_parameter_set_id u(4) ...  Qpc_data_default_flag u(1) if(!Qp_(C)_data_default_flag)  Qp_(C)_data( ) ...

For example, the syntax element Qp_(c)_data_default_flag may representwhether a user defined mode is used for the derivation of a quantizationparameter. For example, Qp_(c)_data_default_flag having a value of 0 mayrepresent that a user defined mode is used for the derivation of aquantization parameter. That is, for example, Qp_(c)_data_default_flaghaving a value of 0 may represent that chroma quantization parameterdata Qp_(c)_data( ) is used. When the Qp_(c)_data_default_flag is 0, thechroma quantization parameter data Qp_(c)_data( ) may be signaled.Furthermore, for example, Qp_(c)_data_default_flag having a value of 1may represent that a default table is used for the derivation of aquantization parameter. The default table may be the same as Table 7.Furthermore, if Qp_(c)_data_default_flag is not present, theQp_(c)_data_default_flag may be inferred as 1.

For example, in the present embodiment, if a process of deriving aquantization parameter is written in a standard format, the process maybe represented as in the following table.

TABLE 41 8.7.1 Derivation process for quantization parameters Inputs tothis process are: -a luma location ( xCb, yCb ) specifying the top-leftluma sample of the current coding block relative to the top-left lumasample of the current picture, -a variable cbWidth specifying the widthof the current coding block in luma samples, -a variable cbHeightspecifying the height of the current coding block in luma samples, -avariable treeType specifying whether a single tree (SINGLE_TREE) or adual tree is used to partition the CTUs and, when a dual tree is used,whether the luma (DUAL_TREE_LUMA) or chroma components(DUAL_TREE_CHROMA) are currently processed. In this process, the lumaquantization parameter Qp′_(Y) and the chroma quantization parametersQp′_(Cb) and Qp′_(Cr) are derived. The luma location ( xQg, yQg ),specifies the top-left luma sample of the current quantization grouprelative to the top left luma sample of the current picture. Thehorizontal and vertical positions xQg and yQg are set equal toCuQgTopLeftX and CuQgTopLeftY, respectively. NOTE - : The currentquantization group is a rectangluar region inside a coding tree blockthat shares the same qP_(Y) _(—) _(PRED). Its width and height are equalto the width and height of the coding tree node of which the top-leftluma sample position is assigned to the variables CuQgTopLeftX andCuQgTopLeftY. When treeType is equal to SINGLE_TREE or DUAL_TREE_LUMA,the predicted luma quantization parameter qP_(Y) _(—) _(PRED) is derivedby the following ordered steps: 1.The variable qP_(Y) _(—) _(PREV) isderived as follows: - If one or more of the following conditions aretrue, qP_(Y) _(—) _(PREV) is set equal to SliceQp_(Y): - The currentquantization group is the first quantization group in a slice. - Thecurrent quantization group is the first quantization group in a brick. -Otherwise, qP_(Y PREV) is set 2. The availability derivation process fora block as specified in clause 6.4.X [Ed. (BB): Neighbouring blocksavailability checking process tbd] is invoked with the location ( xCurr,yCurr ) set equal to ( xCb, yCb ) and the neighbouring location ( xNbY,yNbY ) set equal to ( xQg − 1, yQg ) as inputs, and the output isassigned to availableA. The variable qP_(Y) _(—) _(A) is derived asfollows: - If one or more of the following conditions are true, qP_(Y A)is set equal to qP_(Y PREV): - availableA is equal to FALSE. - the CTBaddress ctbAddrA of the CTB containing the luma coding block coveringthe luma location ( xQg − 1, yQg ) is not equal to CtbAddrInBs, wherectbAddrA is derived as follows: xTmp = ( xQg − 1 ) >> MinTbLog2SizeYyTmp = yQg >> MinTbLog2SizeY minTbAddrA = MinTbAddrZs[ xTmp ][ yTmp ]ctbAddrA = minTbAddrA >> ( 2 * ( CtbLog2SizeY − MinTbLog2SizeY ) ) -Otherwise, qP_(Y) _(—) _(A) is set equal to the luma quantizationparameter Qp_(Y) of the coding unit containing the luma coding blockcovering ( xQg − 1, yQg ). 3. The availability derivation process for ablock as specified in clause 6.4.X [Ed. (BB): Neighbouring blocksavailability checking process tbd] is invoked with the location ( xCurr,yCurr ) set equal to ( xCb, yCb ) and the neighbouring location ( xNbY,yNbY ) set equal to ( xQg, yQg − 1 ) as inputs, and the output isassigned to availableB. The variable qP_(Y) _(—) _(B) is derived asfollows: - If one or more of the following conditions are true, qP_(Y B)is set equal to qP_(Y PREV): - availableB is equal to FALSE. - the CTBaddress ctbAddrB of the CTB containing the luma coding block coveringthe luma location ( xQg, yQg − 1 ) is not equal to CtbAddrInBs, wherectbAddrB is derived as follows: xTmp = xQg >> MinTbLog2SizeY yTmp = (yQg − 1 ) >> MinTbLog2SizeY minTbAddrB = MinTbAddrZs[ xTmp ][ yTmp ]ctbAddrB = minTbAddrB >> ( 2 * ( CtbLog2SizeY − MinTbLog2SizeY ) )(8-922) - Otherwise, qP_(Y B) is set equal to the luma quantizationparameter Qp_(Y) of the coding unit containing the luma coding blockcovering ( xQg, vQg − 1 ). 4. The predicted luma quantization parameterqP_(Y) _(—) _(PRED) is derived as follows: - If all the followingconditions are true, then qP_(Y) _(—) _(PRED) is set equal to the lumaquantization parameter Qp_(Y) of the coding unit containing the lumacoding block covering ( xQg, yQg − 1 ): - availableB is equal to TRUE. -the current quantization group is the first quantization group in a CTBrow within a brick - Otherwise, qP_(Y PRED) is derived as follows:qP_(Y PRED) = ( qP_(Y A) + qP_(Y B) + 1 ) >> 1 The variable Qp_(Y) isderived as follows: QP_(Y) = ( ( qP_(Y) _(—) _(PRED) + CuQpDeltaVal +64 + 2 * QpBdOffset_(Y) )%( 64 + QpBdOffset_(Y)) ) − Qp BdOffset_(Y) Theluma quantization parameter Qp′_(Y) is derived as follows: Qp′_(Y) =Qp_(Y) + QpBdOffset_(Y) When ChromaArrayType is not equal to 0 andtreeType is equal to SINGLE_TREE or DUAL_TREE_CHROMA, the followingapplies: - When treeType is equal to DUAL_TREE_CHROMA, the variableQp_(Y) is set equal to the luma quantization parameter Qp_(Y) of theluma coding unit that covers the luma location ( xCb + cbWidth / 2,yCb + cbHeight / 2 ). - The variables qP_(Cb), qP_(Cr) and qP_(CbCr) arederived as follows: qPi_(Cb) = Clip3( −QpBdOffsetC, 69, Qp_(Y) +pps_cb_qp_offset + slice_cb_qp_offset ) qPi_(Cr) = Clip3( −QpBdOffsetC,69, Qp_(Y) + pps_cr_qp_offset + slice_cr_qp_offset ) qPi_(CbCr) = Clip3(−QpBdOffsetC, 69, Qp_(Y) + pps_joint_cbcr_qp_offset + slice_joint_cbcr_qp_offset ) - If ChromaArrayType is equal to 1, andQpc_data_default_flag is equal to FALSE, the variables qP_(Cb), qP_(Cr)and qP_(CbCr) are set equal to the value of Qp_(C) as-specified inclause 7.x.x based on the index qPi equal to qPi_(Cb), qPi_(Cr) andqPi_(CbCr), respectively. - If ChromaArrayType is equal to 1, andQpc_data_default_flag is equal to TRUE, the variables qP_(Cb), qP_(Cr)and qP_(CbCr) are specified in Table 8-15 based on the index qPi equalto qPi_(Cb), qPi_(Cr) and qP_(CbCr), respectively. - Otherwise, thevariables qP_(Cb), qP_(Cr) and qP_(CbCr) are set equal to Min( qPi, 63), based on the index qPi equal to qPi_(Cb), qPi_(Cr) and qPi_(CbCr),respectively. - The chroma quantization parameters for the Cb and Crcomponents, Qp′_(Cb) and Qp′_(Cr), and joint Cb-Cr coding Qp′_(CbCr) arederived as follows: QP′_(Cb) = qP_(Cb) + QpBdOffset_(C) Qp′_(Cr) =qP_(Cr) + QpBdOffset_(C) QP′_(CbCr) = qP^(CbCr) + QpBdOffset_(C) Table8-15 - Specification of QpC as a function of qPi for ChromaArrayTypeequal to 1 qPi < 30 30 31 32 33 34 35 36 37 38 39 40 41 42 43 > 43Qp_(C) = qPi 29 30 31 32 33 33 34 34 35 35 36 36 37 37 = qPi − 6

Referring to Table 41, when ChromaArrayType is 1 andQp_(c)_data_default_flag indicates false (i.e., for example, whenQp_(c)_data_default_flag is 0), the parameters qP_(Cb), qP_(Cr) andqP_(CbCr) may be derived based on signaled user defined information asproposed in the present embodiment. Furthermore, for example, whenChromaArrayType is 1 and Qp_(c)_data_default_flag indicates true (i.e.,for example, when Qp_(c)_data_default_flag is 1), the parametersqP_(Cb), qP_(Cr) and qP_(CbCr) may be derived by a default table basedon the same indices qPi as qPi_(Cb), qPi_(Cr) and qPi_(CbCr),respectively.

FIG. 4 schematically shows an image encoding method by an encodingapparatus according to the present document. The method disclosed inFIG. 4 may be performed by the encoding apparatus disclosed in FIG. 2.Specifically, for example, S400 to S420 in FIG. 4 may be performed bythe residual processor of the encoding apparatus. S430 may be performedby the entropy encoder of the encoding apparatus. Furthermore, althoughnot illustrated, a process of generating reconstructed samples and areconstructed picture based on residual samples and prediction samplesmay be performed by the adder of the encoding apparatus.

The encoding apparatus derives residual samples for a chroma component(S400). For example, the encoding apparatus may derive the residualsamples by subtracting original samples and prediction samples for acurrent block in a current picture.

Meanwhile, the encoding apparatus may derive the prediction samples ofthe current block based on a prediction mode. In this case, variousprediction methods disclosed in this document, such as inter predictionor intra prediction, may be used.

For example, the encoding apparatus may determine whether to performinter prediction or intra prediction on the current block, and maydetermine a detailed inter prediction mode or a detailed intraprediction mode based on RD costs. The encoding apparatus may deriveprediction samples for the current block based on the determined mode.

The encoding apparatus generates quantization parameter data forcombined chroma coding of the residual samples based on the chroma typeof the chroma component (S410). The encoding apparatus may generate thequantization parameter data for the combined chroma coding of theresidual samples based on a chroma type. In this case, the chroma typemay mean the aforementioned ChromaArrayType. For example, if a value ofthe chroma type is not 0, the encoding apparatus may generate thequantization parameter data for the combined chroma coding. For example,when a value of the chroma type is 1, the decoding apparatus maygenerate the quantization parameter data for the combined chroma coding.In this case, when a value of the chroma type is 0, the chroma type maybe a Monochrome format. When a value of the chroma type is 1, the chromatype may be a 4:2:0 format. When a value of the chroma type is 2, thechroma type may be a 4:2:2 format. When a value of the chroma type is 3,the chroma type may be a 4:4:4 format. Furthermore, the combined chromacoding may also be called joint coding of a chroma component. The chromacomponent may include a Cb component and/or a Cr component.

Furthermore, for example, the encoding apparatus may determine whetherto perform combined chroma coding on residual samples for a chromacomponent based on a chroma type (e.g., when a value of the chroma typeis not 0). If the combined chroma coding is performed on the residualsamples, quantization parameter data for the combined chroma coding ofthe residual samples may be generated. For example, the quantizationparameter data may be signaled through a high level syntax. For example,the quantization parameter data may be signaled through a sequenceparameter set (SPS), a picture parameter set (PPS), a slice header, oran adaptation parameter set (APS).

For example, the quantization parameter data may include a syntaxelement representing a starting index of a chroma quantization parametertable for the combined chroma coding and/or a syntax elementrepresenting a difference between the starting index and last index ofthe chroma quantization parameter table. The syntax element representingthe starting index may be the aforementioned qPi_min_idx. Furthermore,the syntax element representing the difference between the startingindex and the last index may be qPi_delta_max_idx. Furthermore, thechroma quantization parameter table may also be called a chromaquantization parameter mapping table or a user defined quantizationparameter mapping table. Furthermore, the starting index may also becalled a minimum index. Furthermore, for example, the syntax elementrepresenting the starting index and/or the syntax element representingthe difference between the starting index and the last index may besignaled through a high level syntax. For example, the syntax elementrepresenting the starting index and/or the syntax element representingthe difference between the starting index and the last index may besignaled through a sequence parameter set (SPS), a picture parameter set(PPS), a slice header, or an adaptation parameter set (APS).

Furthermore, for example, the quantization parameter data may includesyntax elements for quantization parameter values of indices of thechroma quantization parameter table. That is, for example, thequantization parameter data may include a syntax element for aquantization parameter value of each of indices of the chromaquantization parameter table. The syntax element for the quantizationparameter value of the index may be the aforementionedQp_(C)_qPi_val[i]. Furthermore, for example, the syntax element for thequantization parameter value of the index may be signaled through a highlevel syntax. For example, the syntax element for the quantizationparameter value of the index may be signaled through a sequenceparameter set (SPS), a picture parameter set (PPS), a slice header, oran adaptation parameter set (APS).

Furthermore, for example, the quantization parameter data may include asyntax element representing an offset for the derivation of aquantization parameter for the combined chroma coding. The syntaxelement representing the offset may be the aforementioned QpOffset_(C).Furthermore, for example, the syntax element representing the offset maybe signaled through a high level syntax. For example, the syntax elementrepresenting the offset may be signaled through a sequence parameter set(SPS), a picture parameter set (PPS), a slice header, or an adaptationparameter set (APS).

Meanwhile, for example, the encoding apparatus may derive thequantization parameter for the combined chroma coding based on thequantization parameter data. The quantization parameter for the combinedchroma coding may represent the aforementioned QP′_(CbCr).

For example, as in the aforementioned contents, the chroma quantizationparameter table may be derived based on the syntax element representingthe starting index of the chroma quantization parameter table, thesyntax element representing the difference between the starting indexand last index of the chroma quantization parameter table and/or thesyntax elements for the quantization parameter values of the indices ofthe chroma quantization parameter table. That is, for example, thechroma quantization parameter table for the combined chroma coding maybe derived based on the quantization parameter data. Thereafter, anindex for the combined chroma coding may be derived based on aquantization parameter for a luma component. The quantization parameterfor the combined chroma coding may be derived based on a quantizationparameter for the index of the chroma quantization parameter table. Thatis, for example, the quantization parameter for the combined chromacoding may be derived based on a quantization parameter for the sameindex as the quantization parameter of the luma component in the chromaquantization parameter table.

In addition, for example, the quantization parameter (e.g., QP′_(CbCr))for the combined chroma coding may be derived by adding an offset to thequantization parameter (e.g., QP_(CbCr)) for the index of the chromaquantization parameter table. The offset may be derived based on asyntax element representing an offset for the derivation of thequantization parameter for the combined chroma coding.

The encoding apparatus generates a flag representing whether thequantization parameter data for the combined chroma coding is present(S420). For example, the encoding apparatus may generate whether a flagrepresenting whether the quantization parameter data for the combinedchroma coding is present based on the chroma type. For example, when avalue of the chroma type is not 0, the encoding apparatus may generatethe flag representing whether the quantization parameter data for thecombined chroma coding is present. For example, when a value of thechroma type is 1, the encoding apparatus may generate the flagrepresenting whether the quantization parameter data for the combinedchroma coding is present. For example, the syntax element for the flagmay be the aforementioned Qp_(C)_data_present_flag.

For example, when a value of the flag is 0, the flag may represent thatthe quantization parameter data for the combined chroma coding is notpresent. When a value of the flag is 1, the flag may represent that thequantization parameter data for the combined chroma coding is present.

Furthermore, for example, the flag may be signaled through a high levelsyntax. For example, the flag may be signaled through a sequenceparameter set (SPS), a picture parameter set (PPS), a slice header, oran adaptation parameter set (APS).

The encoding apparatus encodes the quantization parameter data and theflag (S430). The encoding apparatus may encode the quantizationparameter data and the flag. Image information may include thequantization parameter data and the flag.

The encoding apparatus may encode the image information including thequantization parameter data and the flag.

Meanwhile, for example, the encoding apparatus may generate and encodeprediction information for the current block. The prediction informationmay include prediction mode information for the current block. The imageinformation may include the prediction information.

Furthermore, for example, the encoding apparatus may encode residualinformation for the residual samples. The image information may includethe residual information. Furthermore, for example, the encodingapparatus may output the image information in the form of a bitstream byencoding the image information.

Meanwhile, the bitstream including the image information may betransmitted to the decoding apparatus over a network or a (digital)storage medium. In this case, the network may include a broadcastingnetwork and/or a communication network. The digital storage medium mayinclude various storage media, such as a USB, an SD, a CD, a DVD,Blu-ray, an HDD, and an SSD.

FIG. 5 schematically shows an encoding apparatus for performing an imageencoding method according to this document. The method disclosed in FIG.4 may be performed by the encoding apparatus disclosed in FIG. 5.Specifically, for example, the residual processor of the encodingapparatus in FIG. 5 may perform S400 to S420. The entropy encoder of theencoding apparatus may perform S430. Furthermore, although notillustrated, a process of generating reconstructed samples and areconstructed picture based on the residual samples and predictionsamples may be performed by the adder of the encoding apparatus.

FIG. 6 schematically shows an image decoding method by a decodingapparatus according to this document. The method disclosed in FIG. 6 maybe performed by the decoding apparatus disclosed in FIG. 3.Specifically, for example, S600 in FIG. 6 may be performed by theentropy decoder of the decoding apparatus. S610 to S640 in FIG. 6 may beperformed by the residual processor of the decoding apparatus. S650 inFIG. 6 may be performed by the adder of the decoding apparatus.

The decoding apparatus obtains a flag representing whether quantizationparameter data for combined chroma coding is present based on a chromatype (S600). The decoding apparatus may obtain the image informationthrough a bitstream. For example, the image information may includeinformation for a chroma quantization parameter. For example, the imageinformation may include the flag representing whether the quantizationparameter data for the combined chroma coding is present. For example,the decoding apparatus may obtain the flag representing whether thequantization parameter data for the combined chroma coding is presentbased on the chroma type. In this case, the chroma type may mean theaforementioned ChromaArrayType. For example, when a value of the chromatype is not 0, the decoding apparatus may obtain a flag representingwhether quantization parameter data for combined chroma coding ispresent. For example, when a value of the chroma type is 1, the decodingapparatus may obtain a flag representing whether quantization parameterdata for combined chroma coding is present. In this case, when a valueof the chroma type is 0, the chroma type may be a monochrome format.When a value of the chroma type is 1, the chroma type may be a 4:2:0format. When a value of the chroma type is 2, the chroma type may be a4:2:2 format. When a value of the chroma type is 3, the chroma type maybe a 4:4:4 format. Furthermore, the combined chroma coding may also becalled joint coding of a chroma component. The chroma component mayinclude a Cb component and/or a Cr component. For example, a syntaxelement for the flag may be the aforementioned Qp_(C)_data_present_flag.

For example, when a value of the flag is 0, the flag may represent thatthe quantization parameter data for the combined chroma coding is notpresent. When a value of the flag is 1, the flag may represent that thequantization parameter data for the combined chroma coding is present.

Furthermore, for example, the flag may be signaled through a high levelsyntax. For example, the flag may be signaled through a sequenceparameter set (SPS), a picture parameter set (PPS), a slice header, oran adaptation parameter set (APS).

The decoding apparatus obtains the quantization parameter data for thecombined chroma coding based on the flag (S610). The decoding apparatusmay obtain the quantization parameter data for the combined chromacoding based on the flag. For example, the decoding apparatus may obtainthe quantization parameter data for the combined chroma coding based onthe flag representing that the quantization parameter data for thecombined chroma coding is present. That is, for example, when a value ofthe flag is 1, the decoding apparatus may obtain the quantizationparameter data for the combined chroma coding. Furthermore, for example,the quantization parameter data may be signaled through a high levelsyntax. For example, the quantization parameter data may be signaledthrough a sequence parameter set (SPS), a picture parameter set (PPS), aslice header, or an adaptation parameter set (APS).

For example, the quantization parameter data may include a syntaxelement representing a starting index of a chroma quantization parametertable for the combined chroma coding and/or a syntax elementrepresenting a difference between the starting index and last index ofthe chroma quantization parameter table. The syntax element representingthe starting index may be the aforementioned qPi_min_idx. Furthermore,the syntax element representing the difference between the startingindex and the last index may be qPi_delta_max_idx. Furthermore, thechroma quantization parameter table may also be called a chromaquantization parameter mapping table or a user defined quantizationparameter mapping table. Furthermore, the starting index may also becalled a minimum index. Furthermore, for example, the syntax elementrepresenting the starting index and/or the syntax element representingthe difference between the starting index and the last index may besignaled through a high level syntax. For example, the syntax elementrepresenting the starting index and/or the syntax element representingthe difference between the starting index and the last index may besignaled through a sequence parameter set (SPS), a picture parameter set(PPS), a slice header, or an adaptation parameter set (APS).

Furthermore, for example, the quantization parameter data may includesyntax elements for quantization parameter values of indices of thechroma quantization parameter table. That is, for example, thequantization parameter data may include a syntax element for aquantization parameter value of each of indices of the chromaquantization parameter table. The syntax element for the quantizationparameter value of the index may be the aforementionedQp_(C)_qPi_val[i]. Furthermore, for example, the syntax element for thequantization parameter value of the index may be signaled through a highlevel syntax. For example, the syntax element for the quantizationparameter value of the index may be signaled through a sequenceparameter set (SPS), a picture parameter set (PPS), a slice header, oran adaptation parameter set (APS).

Furthermore, for example, the quantization parameter data may include asyntax element representing an offset for the derivation of aquantization parameter for the combined chroma coding. The syntaxelement representing the offset may be the aforementioned QpOffset_(C).Furthermore, for example, the syntax element representing the offset maybe signaled through a high level syntax. For example, the syntax elementrepresenting the offset may be signaled through a sequence parameter set(SPS), a picture parameter set (PPS), a slice header, or an adaptationparameter set (APS).

The decoding apparatus derives a chroma quantization parameter tablebased on the quantization parameter data (S620). The decoding apparatusmay derive a chroma quantization parameter table for the combined chromacoding based on the quantization parameter data. The chroma quantizationparameter table may be called a chroma quantization parameter mappingtable or a user defined quantization parameter mapping table.

For example, as in the aforementioned contents, the chroma quantizationparameter table may be derived based on the syntax element representingthe starting index of the chroma quantization parameter table, thesyntax element representing the difference between the starting indexand last index of the chroma quantization parameter table and/or thesyntax elements for the quantization parameter values of the indices ofthe chroma quantization parameter table. That is, for example, thechroma quantization parameter table for the combined chroma coding maybe derived based on the quantization parameter data.

The decoding apparatus derives the quantization parameter for thecombined chroma coding based on the chroma quantization parameter table(S630). The quantization parameter for the combined chroma coding mayrepresent the aforementioned QP′_(CbCr).

For example, an index for the combined chroma coding may be derivedbased on a quantization parameter for a luma component. The quantizationparameter for the combined chroma coding may be derived based on aquantization parameter for the index of the chroma quantizationparameter table. That is, for example, the quantization parameter forthe combined chroma coding may be derived based on the quantizationparameter for the same index as the quantization parameter of the lumacomponent in the chroma quantization parameter table.

Furthermore, for example, the quantization parameter (e.g., QP′_(CbCr))for the combined chroma coding may be derived by adding an offset to thequantization parameter (e.g., QP_(CbCr)) for the index of the chromaquantization parameter table. The offset may be derived based on asyntax element representing an offset for the derivation of aquantization parameter for the combined chroma coding.

The decoding apparatus derives residual samples based on thequantization parameter (S640). The decoding apparatus may derive theresidual samples based on the quantization parameter. For example, thedecoding apparatus may derive transform coefficients based on residualinformation, may derive the residual samples by dequantizing thetransform coefficients. Alternatively, for example, the decodingapparatus may derive transform coefficients based on residualinformation, may derive inverse-transformed transform coefficients byinverse-transforming the transform coefficients, and may derive theresidual samples by dequantizing the inverse-transformed transformcoefficients.

The decoding apparatus generates a reconstructed picture based on theresidual samples (S650). For example, the decoding apparatus maygenerate the reconstructed picture based on the residual samples.

Meanwhile, for example, the decoding apparatus may derive predictionsamples by performing an inter prediction mode or an intra predictionmode on the current block based on prediction information receivedthrough a bitstream, and may generate the reconstructed samples byadding the prediction samples and the residual samples.

Thereafter, if necessary, in order to improve subjective/objectivepicture quality, an in-loop filtering procedure, such as deblockingfiltering, an SAO and/or an ALF procedure, may be applied to thereconstructed samples as described above.

FIG. 7 schematically illustrates the decoding apparatus performing theimage decoding method according to this document. The method disclosedin FIG. 6 may be performed by the decoding apparatus disclosed in FIG.7. Specifically, for example, the entropy decoder of the decodingapparatus of FIG. 7 may perform S600 in FIG. 6. The residual processorof the decoding apparatus of FIG. 7 may perform S610 to S640 of FIG. 6.The adder of the decoding apparatus of FIG. 7 may perform S650 in FIG.6.

According to the present disclosure, the chroma quantization parametertable for quantization parameter derivation can be determined based on aflag representing whether to transmit quantization parameter data forquantization parameter derivation for a chroma component. Codingefficiency can be improved by performing coding based on a quantizationparameter according to characteristics of an image.

Furthermore, according to the present disclosure, a chroma quantizationparameter table for a chroma component can be determined based onsignaled chroma quantization data. Coding efficiency can be improved byperforming coding based on a quantization parameter according tocharacteristics of an image.

In the above-described embodiment, the methods are described based onthe flowchart having a series of steps or blocks. The present disclosureis not limited to the order of the above steps or blocks. Some steps orblocks may occur simultaneously or in a different order from other stepsor blocks as described above. Further, those skilled in the art willunderstand that the steps shown in the above flowchart are notexclusive, that further steps may be included, or that one or more stepsin the flowchart may be deleted without affecting the scope of thepresent disclosure.

The embodiments described in this specification may be performed bybeing implemented on a processor, a microprocessor, a controller or achip. For example, the functional units shown in each drawing may beperformed by being implemented on a computer, a processor, amicroprocessor, a controller or a chip. In this case, information forimplementation (e.g., information on instructions) or algorithm may bestored in a digital storage medium.

In addition, the decoding apparatus and the encoding apparatus to whichthe present disclosure is applied may be included in a multimediabroadcasting transmission/reception apparatus, a mobile communicationterminal, a home cinema video apparatus, a digital cinema videoapparatus, a surveillance camera, a video chatting apparatus, areal-time communication apparatus such as video communication, a mobilestreaming apparatus, a storage medium, a camcorder, a VoD serviceproviding apparatus, an Over the top (OTT) video apparatus, an Internetstreaming service providing apparatus, a three-dimensional (3D) videoapparatus, a teleconference video apparatus, a transportation userequipment (e.g., vehicle user equipment, an airplane user equipment, aship user equipment, etc.) and a medical video apparatus and may be usedto process video signals and data signals. For example, the Over the top(OTT) video apparatus may include a game console, a blue-ray player, aninternet access TV, a home theater system, a smart phone, a tablet PC, aDigital Video Recorder (DVR), and the like.

Furthermore, the processing method to which the present disclosure isapplied may be produced in the form of a program that is to be executedby a computer and may be stored in a computer-readable recording medium.Multimedia data having a data structure according to the presentdisclosure may also be stored in computer-readable recording media. Thecomputer-readable recording media include all types of storage devicesin which data readable by a computer system is stored. Thecomputer-readable recording media may include a BD, a Universal SerialBus (USB), ROM, PROM, EPROM, EEPROM, RAM, CD-ROM, a magnetic tape, afloppy disk, and an optical data storage device, for example.Furthermore, the computer-readable recording media includes mediaimplemented in the form of carrier waves (e.g., transmission through theInternet). In addition, a bit stream generated by the encoding methodmay be stored in a computer-readable recording medium or may betransmitted over wired/wireless communication networks.

In addition, the embodiments of the present disclosure may beimplemented with a computer program product according to program codes,and the program codes may be performed in a computer by the embodimentsof the present disclosure. The program codes may be stored on a carrierwhich is readable by a computer.

FIG. 8 illustrates a structural diagram of a contents streaming systemto which the present disclosure is applied.

The content streaming system to which the embodiment(s) of the presentdisclosure is applied may largely include an encoding server, astreaming server, a web server, a media storage, a user device, and amultimedia input device.

The encoding server compresses content input from multimedia inputdevices such as a smartphone, a camera, a camcorder, etc. Into digitaldata to generate a bitstream and transmit the bitstream to the streamingserver. As another example, when the multimedia input devices such assmartphones, cameras, camcorders, etc. directly generate a bitstream,the encoding server may be omitted.

The bitstream may be generated by an encoding method or a bitstreamgenerating method to which the embodiment(s) of the present disclosureis applied, and the streaming server may temporarily store the bitstreamin the process of transmitting or receiving the bitstream.

The streaming server transmits the multimedia data to the user devicebased on a user's request through the web server, and the web serverserves as a medium for informing the user of a service. When the userrequests a desired service from the web server, the web server deliversit to a streaming server, and the streaming server transmits multimediadata to the user. In this case, the content streaming system may includea separate control server. In this case, the control server serves tocontrol a command/response between devices in the content streamingsystem.

The streaming server may receive content from a media storage and/or anencoding server. For example, when the content is received from theencoding server, the content may be received in real time. In this case,in order to provide a smooth streaming service, the streaming server maystore the bitstream for a predetermined time.

Examples of the user device may include a mobile phone, a smartphone, alaptop computer, a digital broadcasting terminal, a personal digitalassistant (PDA), a portable multimedia player (PMP), navigation, a slatePC, tablet PCs, ultrabooks, wearable devices (ex. Smartwatches, smartglasses, head mounted displays), digital TVs, desktops computer, digitalsignage, and the like. Each server in the content streaming system maybe operated as a distributed server, in which case data received fromeach server may be distributed.

The claims described in the present disclosure may be combined invarious ways. For example, the technical features of the method claimsof the present disclosure may be combined to be implemented as anapparatus, and the technical features of the apparatus claims of thepresent disclosure may be combined to be implemented as a method. Inaddition, the technical features of the method claim of the presentdisclosure and the technical features of the apparatus claim may becombined to be implemented as an apparatus, and the technical featuresof the method claim of the present disclosure and the technical featuresof the apparatus claim may be combined to be implemented as a method.

What is claimed is:
 1. An image decoding method, performed by a decodingapparatus, comprising: receiving a flag representing whetherquantization parameter data for combined chroma coding is present, basedon a chroma type; receiving the quantization parameter data for thecombined chroma coding based on the flag; receiving quantizationparameter offset information for the combined chroma coding; deriving achroma quantization parameter table based on the quantization parameterdata; deriving a quantization parameter for the combined chroma codingbased on the chroma quantization parameter table and the quantizationparameter offset information; deriving residual samples based on thequantization parameter; and generating a reconstructed picture based onthe residual samples, wherein the quantization parameter data includes asyntax element for a starting quantization parameter of the chromaquantization parameter table and a syntax element for a number ofquantization parameters in the chroma quantization parameter table. 2.The image decoding method of claim 1, wherein when a value of the flagis 1, the flag represents that the quantization parameter data for thecombined chroma coding is present.
 3. The image decoding method of claim1, wherein the flag is signaled through a sequence parameter set (SPS).4. The image decoding method of claim 1, wherein the quantizationparameter data includes syntax elements for quantization parametervalues of indices of the chroma quantization parameter table.
 5. Theimage decoding method of claim 1, wherein a value of the chroma type isnot
 0. 6. The image decoding method of claim 1, wherein the quantizationparameter for the combined chroma coding is derived based on aquantization parameter for an index identical with an index of aquantization parameter for a luma component corresponding to the currentchroma block.
 7. The image decoding method of claim 1, wherein thequantization parameter offset information is signaled through a pictureparameter set (PPS).
 8. An image encoding method, performed by anencoding apparatus, comprising: deriving residual samples for a chromacomponent; generating quantization parameter data for combined chromacoding of the residual samples based on a chroma type of the chromacomponent; generating quantization parameter offset information for thecombined chroma coding; generating a flag representing whether thequantization parameter data for the combined chroma coding is present;and generating a bitstream including the quantization parameter data,quantization parameter offset information and the flag, wherein thequantization parameter data includes a syntax element for a startingquantization parameter of a chroma quantization parameter table and asyntax element for a number of quantization parameters in the chromaquantization parameter table.
 9. The image encoding method of claim 8,wherein when a value of the flag is 1, the flag represents that thequantization parameter data for the combined chroma coding is present.10. The image encoding method of claim 8, wherein the flag is signaledthrough a sequence parameter set (SPS).
 11. The image encoding method ofclaim 8, wherein the quantization parameter data includes syntaxelements for quantization parameter values of indices of the chromaquantization parameter table.
 12. The image encoding method of claim 11,wherein the chroma quantization parameter table for the combined chromacoding is derived based on the quantization parameter data, wherein aquantization parameter for the combined chroma coding is derived basedon the chroma quantization parameter table.
 13. A non-transitorycomputer-readable storage medium storing a bitstream generated by amethod, the method comprising: deriving residual samples for a chromacomponent; generating quantization parameter data for combined chromacoding of the residual samples based on a chroma type of the chromacomponent; generating quantization parameter offset information for thecombined chroma coding; generating a flag representing whether thequantization parameter data for the combined chroma coding is present;and generating the bitstream including the quantization parameter data,quantization parameter offset information and the flag, wherein thequantization parameter data includes a syntax element for a startingquantization parameter of a chroma quantization parameter table and asyntax element for a number of quantization parameters in the chromaquantization parameter table.